Lock owned sequences during ALTER TABLE SET { LOGGED | UNLOGGED }.

nmisch · nmisch · commit 24561b498f11 · 2024-06-27T19:21:10.000-07:00
These commands already make the persistence of owned sequences follow owned table persistence changes. They didn't lock those sequences. They lost the effect of nextval() calls that other sessions make after the ALTER TABLE command, before the ALTER TABLE transaction commits. Fix by acquiring the same lock that ALTER SEQUENCE SET { LOGGED | UNLOGGED } acquires. This might cause more deadlocks. Back-patch to v15, where commit 344d62f introduced unlogged sequences. Reviewed (in an earlier version) by Robert Haas. Discussion: https://postgr.es/m/20240611024525.9f.nmisch@google.com
diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
@@ -558,6 +558,13 @@ SequenceChangePersistence(Oid relid, char newrelpersistence)
 	Buffer		buf;
 	HeapTupleData seqdatatuple;
 
+	/*
+	 * ALTER SEQUENCE acquires this lock earlier.  If we're processing an
+	 * owned sequence for ALTER TABLE, lock now.  Without the lock, we'd
+	 * discard increments from nextval() calls (in other sessions) between
+	 * this function's buffer unlock and this transaction's commit.
+	 */
+	LockRelationOid(relid, AccessExclusiveLock);
 	init_sequence(relid, &elm, &seqrel);
 
 	/* check the comment above nextval_internal()'s equivalent call. */
