Subject: [PATCH] Test to demonstrate bug in commit 0d3dba38c7 and to verify a fix.

From: Antonin Houska <[email-masked]>
Date: Tue, 12 May 2026 12:27:08 +0200

The fix: https://www.postgresql.org/message-id/77611.1778055944%40localhost
---
 src/backend/access/transam/xact.c             |   2 +
 src/backend/commands/repack_worker.c          |   2 +
 src/test/isolation/isolationtester.c          |   9 +-
 .../expected/repack_running_xacts.out         |  81 ++++++++++++
 .../specs/repack_running_xacts.spec           | 119 ++++++++++++++++++
 5 files changed, 212 insertions(+), 1 deletion(-)
 create mode 100644 src/test/modules/injection_points/expected/repack_running_xacts.out
 create mode 100644 src/test/modules/injection_points/specs/repack_running_xacts.spec

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 5586fbe5b07..b63ee166028 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -65,6 +65,7 @@
 #include "utils/builtins.h"
 #include "utils/combocid.h"
 #include "utils/guc.h"
+#include "utils/injection_point.h"
 #include "utils/inval.h"
 #include "utils/memutils.h"
 #include "utils/relmapper.h"
@@ -2428,6 +2429,7 @@ CommitTransaction(void)
 	 * must be done _before_ releasing locks we hold and _after_
 	 * RecordTransactionCommit.
 	 */
+	INJECTION_POINT("before-end-transaction", NULL);
 	ProcArrayEndTransaction(MyProc, latestXid);
 
 	/*
diff --git a/src/backend/commands/repack_worker.c b/src/backend/commands/repack_worker.c
index c40f8c98e06..6835626d677 100644
--- a/src/backend/commands/repack_worker.c
+++ b/src/backend/commands/repack_worker.c
@@ -26,6 +26,7 @@
 #include "storage/ipc.h"
 #include "storage/proc.h"
 #include "tcop/tcopprot.h"
+#include "utils/injection_point.h"
 #include "utils/memutils.h"
 
 #define REPL_PLUGIN_NAME   "pgrepack"
@@ -233,6 +234,7 @@ repack_setup_logical_decoding(Oid relid)
 	 * Neither prepare_write nor do_write callback nor update_progress is
 	 * useful for us.
 	 */
+	INJECTION_POINT("before-create-decoding-context", NULL);
 	ctx = CreateInitDecodingContext(REPL_PLUGIN_NAME,
 									NIL,
 									true,
diff --git a/src/test/isolation/isolationtester.c b/src/test/isolation/isolationtester.c
index 440c875b8ac..8f17ee412c9 100644
--- a/src/test/isolation/isolationtester.c
+++ b/src/test/isolation/isolationtester.c
@@ -216,15 +216,22 @@ main(int argc, char **argv)
 	 * exactly expect concurrent use of test tables.  However, autovacuum will
 	 * occasionally take AccessExclusiveLock to truncate a table, and we must
 	 * ignore that transient wait.
+	 *
+	 * If the session's backend is blocked, and if its background worker is
+	 * waiting on an injection point, we assume that the injection point is
+	 * the reason for the backend to be blocked. That's what we check in the
+	 * second query of the UNION. XXX Should we use a separate query for that?
 	 */
 	initPQExpBuffer(&wait_query);
 	appendPQExpBufferStr(&wait_query,
+						 "WITH blocking(res) AS ("
 						 "SELECT pg_catalog.pg_isolation_test_session_is_blocked($1, '{");
 	/* The spec syntax requires at least one session; assume that here. */
 	appendPQExpBufferStr(&wait_query, conns[1].backend_pid_str);
 	for (i = 2; i < nconns; i++)
 		appendPQExpBuffer(&wait_query, ",%s", conns[i].backend_pid_str);
-	appendPQExpBufferStr(&wait_query, "}')");
+	appendPQExpBufferStr(&wait_query, "}') UNION "
+						 "SELECT pg_catalog.pg_isolation_test_session_is_blocked(pid, '{}') FROM pg_stat_activity WHERE leader_pid=$1) SELECT bool_or(res) FROM blocking");
 
 	res = PQprepare(conns[0].conn, PREP_WAITING, wait_query.data, 0, NULL);
 	if (PQresultStatus(res) != PGRES_COMMAND_OK)
diff --git a/src/test/modules/injection_points/expected/repack_running_xacts.out b/src/test/modules/injection_points/expected/repack_running_xacts.out
new file mode 100644
index 00000000000..271fe2b97cb
--- /dev/null
+++ b/src/test/modules/injection_points/expected/repack_running_xacts.out
@@ -0,0 +1,81 @@
+Parsed test spec with 5 sessions
+
+starting permutation: repack s3_assign_xid wakeup_bcdc s4_changes s4_attach s4_commit s3_commit s5_assign_xid wakeup_bet s5_commit check
+injection_points_attach
+-----------------------
+                       
+(1 row)
+
+step repack: 
+	REPACK (CONCURRENTLY) repack_test;
+ <waiting ...>
+step s3_assign_xid: 
+	BEGIN;
+	INSERT INTO aux VALUES (1);
+
+step wakeup_bcdc: 
+	SELECT injection_points_wakeup('before-create-decoding-context');
+
+injection_points_wakeup
+-----------------------
+                       
+(1 row)
+
+step s4_changes: 
+	BEGIN;
+	INSERT INTO repack_test(i, j) VALUES (1, 1);
+
+step s4_attach: 
+	SELECT injection_points_set_local();
+	SELECT injection_points_attach('before-end-transaction', 'wait');
+
+injection_points_set_local
+--------------------------
+                          
+(1 row)
+
+injection_points_attach
+-----------------------
+                       
+(1 row)
+
+step s4_commit: 
+	COMMIT;
+ <waiting ...>
+step s3_commit: 
+	COMMIT;
+
+step s5_assign_xid: 
+	BEGIN;
+	INSERT INTO aux VALUES (2);
+
+step wakeup_bet: 
+	SELECT injection_points_wakeup('before-end-transaction');
+
+injection_points_wakeup
+-----------------------
+                       
+(1 row)
+
+step repack: <... completed>
+step s4_commit: <... completed>
+step s5_commit: 
+	COMMIT;
+
+step check: 
+	TABLE repack_test;
+
+i|j
+-+-
+(0 rows)
+
+injection_points_detach
+-----------------------
+                       
+(1 row)
+
+injection_points_detach
+-----------------------
+                       
+(1 row)
+
diff --git a/src/test/modules/injection_points/specs/repack_running_xacts.spec b/src/test/modules/injection_points/specs/repack_running_xacts.spec
new file mode 100644
index 00000000000..1f878514046
--- /dev/null
+++ b/src/test/modules/injection_points/specs/repack_running_xacts.spec
@@ -0,0 +1,119 @@
+setup
+{
+	CREATE EXTENSION injection_points;
+	CREATE TABLE repack_test(i int PRIMARY KEY, j int);
+	CREATE TABLE aux(i int);
+}
+
+teardown
+{
+	DROP TABLE repack_test;
+	DROP TABLE aux;
+	DROP EXTENSION injection_points;
+}
+
+session s1
+setup
+{
+	SELECT injection_points_attach('before-create-decoding-context', 'wait');
+}
+step repack
+{
+	REPACK (CONCURRENTLY) repack_test;
+}
+step check
+{
+	TABLE repack_test;
+}
+teardown
+{
+	SELECT injection_points_detach('before-create-decoding-context');
+}
+
+session s2
+step wakeup_bcdc
+{
+	SELECT injection_points_wakeup('before-create-decoding-context');
+}
+step wakeup_bet
+{
+	SELECT injection_points_wakeup('before-end-transaction');
+}
+
+session s3
+step s3_assign_xid
+{
+	BEGIN;
+	INSERT INTO aux VALUES (1);
+}
+step s3_commit
+{
+	COMMIT;
+}
+
+session s4
+step s4_changes
+{
+	BEGIN;
+	INSERT INTO repack_test(i, j) VALUES (1, 1);
+}
+# Do not attach in the setup section, that would be too soon.
+step s4_attach
+{
+	SELECT injection_points_set_local();
+	SELECT injection_points_attach('before-end-transaction', 'wait');
+}
+step s4_commit
+{
+	COMMIT;
+}
+teardown
+{
+	SELECT injection_points_detach('before-end-transaction');
+}
+
+session s5
+step s5_assign_xid
+{
+	BEGIN;
+	INSERT INTO aux VALUES (2);
+}
+step s5_commit
+{
+	COMMIT;
+}
+
+permutation
+repack
+# Assign XID so that a running transaction prevents the snapshot builder from
+# reaching CONSISTENT state immediately. It will wait for this to complete
+# after having reached BUILDING_SNAPSHOT.
+s3_assign_xid
+# Let the decoding setup start.
+wakeup_bcdc
+# Likewise, the snapshot builder will wait for the s4's xact to complete after
+# having reached FULL_SNAPSHOT. This is the problematic transaction, so let it
+# do some changes.
+s4_changes
+# Attach to the 'before-end-transaction' injection point that s4 will need
+# during commit.
+s4_attach
+# Only write commit record for s4, but do not remove the xact from procarray
+# yet. Thus the snapshot builder still needs to wait.
+s4_commit
+# Let the snapshot builder proceed to FULL_SNAPSHOT.
+s3_commit
+# Start another transaction so that CONSISTENT is not reached "directly",
+# i.e. due to no running transaction. It's important here that builder->xmin
+# does not advance.
+s5_assign_xid
+# Remove s4 xact from procarray, and thus reach the CONSISTENT state. Since
+# the COMMIT appeared in WAL too early (i.e. when the snapshot builder state
+# did not allow decoding of COMMIT records yet), the snapshot builder will
+# consider s4 running. This is also due to returning from
+# SnapBuildProcessRunningXacts() too early, w/o advancing builder->xmin.
+wakeup_bet
+# s5 is not needed anymore
+s5_commit
+# Show that the data changes performed by s4 are lost.
+check
-- 
2.47.3

--=-=-=--