Fix snapshot used in logical replication index lookup

hlinnaka · hlinnaka · commit 8171d2dae93d · 2025-03-10T17:08:02.000+02:00
The function calls GetLatestSnapshot() to acquire a fresh snapshot, makes it active, and was meant to pass it to table_tuple_lock(), but instead called GetLatestSnapshot() again to acquire yet another snapshot. It was harmless because the heap AM and all other known table AMs ignore the 'snapshot' argument anyway, but let's be tidy. In the long run, this perhaps should be redesigned so that snapshot was not needed in the first place. The table AM API uses TID + snapshot as the unique identifier for the row version, which is questionable when the row came from an index scan with a Dirty snapshot. You might lock a different row version when you use a different snapshot in the table_tuple_lock() call (a fresh MVCC snapshot) than in the index scan (DirtySnapshot). However, in the heap AM and other AMs where the TID alone identifies the row version, it doesn't matter. So for now, just fix the obvious albeit harmless bug. This has been wrong ever since the table AM API was introduced in commit 5db6df0, so backpatch to all supported versions. Discussion: https://www.postgresql.org/message-id/83d243d6-ad8d-4307-8b51-2ee5844f6230@iki.fi Backpatch-through: 13
diff --git a/src/backend/executor/execReplication.c b/src/backend/executor/execReplication.c
@@ -209,7 +209,7 @@ RelationFindReplTupleByIndex(Relation rel, Oid idxoid,
 
 		PushActiveSnapshot(GetLatestSnapshot());
 
-		res = table_tuple_lock(rel, &(outslot->tts_tid), GetLatestSnapshot(),
+		res = table_tuple_lock(rel, &(outslot->tts_tid), GetActiveSnapshot(),
 							   outslot,
 							   GetCurrentCommandId(false),
 							   lockmode,
