postgres.git / summary / log / commit / refs

commit    29f9fb8fe87005b2a75bd8ad2bc3d1f3ac0e9561
Author:   Alvaro Herrera <alvherre@alvh.no-ip.org>
Date:     Tue Jan 04 16:01:05 2022 +0000
First released in: 15.0, 10.20, 11.15, 12.10, 13.6, 14.2

    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 94edb1decae..827c43e0ccb 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -4497,7 +4497,15 @@ failed:
 	{
 		Assert(result == TM_SelfModified || result == TM_Updated ||
 			   result == TM_Deleted || result == TM_WouldBlock);
-		Assert(!(tuple->t_data->t_infomask & HEAP_XMAX_INVALID));
+
+		/*
+		 * When locking a tuple under LockWaitSkip semantics and we fail with
+		 * TM_WouldBlock 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(TM_WouldBlock || !(tuple->t_data->t_infomask & HEAP_XMAX_INVALID));
 		Assert(result != TM_Updated ||
 			   !ItemPointerEquals(&tuple->t_self, &tuple->t_data->t_ctid));
 		tmfd->ctid = tuple->t_data->t_ctid;


[parent: 20d08b2c61cd]