Add missing buffer lock acquisition in GetTupleForTrigger().

If we had not been holding buffer pin continuously since the tuple was
initially fetched by the UPDATE or DELETE query, it would be possible for
VACUUM or a page-prune operation to move the tuple while we're trying to
copy it.  This would result in a garbage "old" tuple value being passed to
an AFTER ROW UPDATE or AFTER ROW DELETE trigger.  The preconditions for
this are somewhat improbable, and the timing constraints are very tight;
so it's not so surprising that this hasn't been reported from the field,
even though the bug has been there a long time.

Problem found by Andres Freund.  Back-patch to all active branches.

src/backend/commands/trigger.c

@@ -2447,6 +2447,16 @@ ltrmark:;
 
 		buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
 
+		/*
+		 * Although we already know this tuple is valid, we must lock the
+		 * buffer to ensure that no one has a buffer cleanup lock; otherwise
+		 * they might move the tuple while we try to copy it.  But we can
+		 * release the lock before actually doing the heap_copytuple call,
+		 * since holding pin is sufficient to prevent anyone from getting a
+		 * cleanup lock they don't already hold.
+		 */
+		LockBuffer(buffer, BUFFER_LOCK_SHARE);
+
 		page = BufferGetPage(buffer);
 		lp = PageGetItemId(page, ItemPointerGetOffsetNumber(tid));
 
@@ -2456,6 +2466,8 @@ ltrmark:;
 		tuple.t_len = ItemIdGetLength(lp);
 		tuple.t_self = *tid;
 		tuple.t_tableOid = RelationGetRelid(relation);
+
+		LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 	}
 
 	result = heap_copytuple(&tuple);