From 026a93727c85704cb885a2d83c87a576b4fe2c86 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Tue, 4 Jan 2022 13:01:05 -0300
Subject: [PATCH] Allow special SKIP LOCKED condition in Assert()
Under concurrency, it is possible for two sessions to be merrily locking
and releasing a tuple and marking it again as HEAP_XMAX_INVALID all the
while a third session attempts to lock it, miserably fails at it, and
then contemplates life, the universe and everything only to eventually
fail an assertion that said bit is not set. Before SKIP LOCKED that was
indeed a reasonable expectation, but alas! commit df630b0dd5ea falsified
it.
This bug is as old as time itself, and even older, if you think time
begins with the oldest supported branch. Therefore, backpatch to all
supported branches.
Author: Simon Riggs <simon.riggs@enterprisedb.com>
Discussion: https://postgr.es/m/CANbhV-FeEwMnN8yuMyss7if1ZKjOKfjcgqB26n8pqu1e=q0ebg@mail.gmail.com
---
src/backend/access/heap/heapam.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 25fcd4c5241..922378feddc 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -5087,7 +5087,15 @@ failed:
{
Assert(result == HeapTupleSelfUpdated || result == HeapTupleUpdated ||
result == HeapTupleWouldBlock);
- Assert(!(tuple->t_data->t_infomask & HEAP_XMAX_INVALID));
+
+ /*
+ * When locking a tuple under LockWaitSkip semantics and we fail with
+ * HeapTupleWouldBlock above, it's possible for concurrent transactions
+ * to release the lock and set HEAP_XMAX_INVALID in the meantime. So
+ * this assert is slightly different from the equivalent one in
+ * heap_delete and heap_update.
+ */
+ Assert(HeapTupleWouldBlock || !(tuple->t_data->t_infomask & HEAP_XMAX_INVALID));
hufd->ctid = tuple->t_data->t_ctid;
hufd->xmax = HeapTupleHeaderGetUpdateXid(tuple->t_data);
if (result == HeapTupleSelfUpdated)
--
2.39.5