Unpin buffer before inplace update waits for an XID to end.

Commit a07e03fd8fa7daf4d1356f7cb501ffe784ea6257 changed inplace updates
to wait for heap_update() commands like GRANT TABLE and GRANT DATABASE.
By keeping the pin during that wait, a sequence of autovacuum workers
and an uncommitted GRANT starved one foreground LockBufferForCleanup()
for six minutes, on buildfarm member sarus.  Prevent, at the cost of a
bit of complexity.  Back-patch to v12, like the earlier commit.  That
commit and heap_inplace_lock() have not yet appeared in any release.

Discussion: https://postgr.es/m/20241026184936.ae.nmisch@google.com

1 files changed, 5 insertions, 7 deletions

diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
index 085f783a0bc..bd63bf617ad 100644
--- a/src/backend/access/index/genam.c
+++ b/src/backend/access/index/genam.c
@@ -708,6 +708,7 @@ systable_inplace_update_begin(Relation relation,
 	int			retries = 0;
 	SysScanDesc scan;
 	HeapTuple	oldtup;
+	BufferHeapTupleTableSlot *bslot;
 
 	/*
 	 * For now, we don't allow parallel updates.  Unlike a regular update,
@@ -729,10 +730,9 @@ systable_inplace_update_begin(Relation relation,
 	Assert(IsInplaceUpdateRelation(relation) || !IsSystemRelation(relation));
 
 	/* Loop for an exclusive-locked buffer of a non-updated tuple. */
-	for (;;)
+	do
 	{
 		TupleTableSlot *slot;
-		BufferHeapTupleTableSlot *bslot;
 
 		CHECK_FOR_INTERRUPTS();
 
@@ -758,11 +758,9 @@ systable_inplace_update_begin(Relation relation,
 		slot = scan->slot;
 		Assert(TTS_IS_BUFFERTUPLE(slot));
 		bslot = (BufferHeapTupleTableSlot *) slot;
-		if (heap_inplace_lock(scan->heap_rel,
-							  bslot->base.tuple, bslot->buffer))
-			break;
-		systable_endscan(scan);
-	};
+	} while (!heap_inplace_lock(scan->heap_rel,
+								bslot->base.tuple, bslot->buffer,
+								(void (*) (void *)) systable_endscan, scan));
 
 	*oldtupcopy = heap_copytuple(oldtup);
 	*state = scan;