For inplace update durability, make heap_update() callers wait.

The previous commit fixed some ways of losing an inplace update.  It
remained possible to lose one when a backend working toward a
heap_update() copied a tuple into memory just before inplace update of
that tuple.  In catalogs eligible for inplace update, use LOCKTAG_TUPLE
to govern admission to the steps of copying an old tuple, modifying it,
and issuing heap_update().  This includes MERGE commands.  To avoid
changing most of the pg_class DDL, don't require LOCKTAG_TUPLE when
holding a relation lock sufficient to exclude inplace updaters.
Back-patch to v12 (all supported versions).  In v13 and v12, "UPDATE
pg_class" or "UPDATE pg_database" can still lose an inplace update.  The
v14+ UPDATE fix needs commit 86dc90056dfdbd9d1b891718d2e5614e3e432f35,
and it wasn't worth reimplementing that fix without such infrastructure.

Reviewed by Nitin Motiani and (in earlier versions) Heikki Linnakangas.

Discussion: https://postgr.es/m/20231027214946.79.nmisch@google.com

1 files changed, 5 insertions, 2 deletions

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 5f533dbcbcc..e3c61cd907c 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -3781,14 +3781,17 @@ update_relispartition(Oid relationId, bool newval)
 {
 	HeapTuple	tup;
 	Relation	classRel;
+	ItemPointerData otid;
 
 	classRel = table_open(RelationRelationId, RowExclusiveLock);
-	tup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relationId));
+	tup = SearchSysCacheLockedCopy1(RELOID, ObjectIdGetDatum(relationId));
 	if (!HeapTupleIsValid(tup))
 		elog(ERROR, "cache lookup failed for relation %u", relationId);
+	otid = tup->t_self;
 	Assert(((Form_pg_class) GETSTRUCT(tup))->relispartition != newval);
 	((Form_pg_class) GETSTRUCT(tup))->relispartition = newval;
-	CatalogTupleUpdate(classRel, &tup->t_self, tup);
+	CatalogTupleUpdate(classRel, &otid, tup);
+	UnlockTuple(classRel, &otid, InplaceUpdateTupleLock);
 	heap_freetuple(tup);
 	table_close(classRel, RowExclusiveLock);
 }