Author:     Noah Misch <noah@leadboat.com>
Commit:     Noah Misch <noah@leadboat.com>

    Lock owned sequences during ALTER TABLE SET { LOGGED | UNLOGGED }.
    
    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 344d62fb9a978a72cf8347f0369b9ee643fd0b31 introduced
    unlogged sequences.
    
    Reviewed by FIXME.
    
    Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index 28f8522..b4ad19c 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -545,6 +545,13 @@ 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. */