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/[email protected]

1 files changed, 5 insertions, 2 deletions

diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index 55baf10c856..05a6de68ba3 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -388,6 +388,7 @@ SetDatabaseHasLoginEventTriggers(void)
 	/* Set dathasloginevt flag in pg_database */
 	Form_pg_database db;
 	Relation	pg_db = table_open(DatabaseRelationId, RowExclusiveLock);
+	ItemPointerData otid;
 	HeapTuple	tuple;
 
 	/*
@@ -399,16 +400,18 @@ SetDatabaseHasLoginEventTriggers(void)
 	 */
 	LockSharedObject(DatabaseRelationId, MyDatabaseId, 0, AccessExclusiveLock);
 
-	tuple = SearchSysCacheCopy1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
+	tuple = SearchSysCacheLockedCopy1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
 	if (!HeapTupleIsValid(tuple))
 		elog(ERROR, "cache lookup failed for database %u", MyDatabaseId);
+	otid = tuple->t_self;
 	db = (Form_pg_database) GETSTRUCT(tuple);
 	if (!db->dathasloginevt)
 	{
 		db->dathasloginevt = true;
-		CatalogTupleUpdate(pg_db, &tuple->t_self, tuple);
+		CatalogTupleUpdate(pg_db, &otid, tuple);
 		CommandCounterIncrement();
 	}
+	UnlockTuple(pg_db, &otid, InplaceUpdateTupleLock);
 	table_close(pg_db, RowExclusiveLock);
 	heap_freetuple(tuple);
 }