On Mon, Nov 21, 2022 at 4:31 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Sat, Nov 19, 2022 at 6:35 AM Andres Freund <and...@anarazel.de> wrote: > > > > On 2022-11-18 11:20:36 +0530, Amit Kapila wrote: > > > Okay, updated the patch accordingly. > > > > Assuming it passes tests etc, this'd work for me. > > > > Thanks, Pushed.
The same assertion failure has been reported on another thread[1].
Since I could reproduce this issue several times in my environment
I've investigated the root cause.
I think there is a race condition of updating
procArray->replication_slot_xmin by CreateInitDecodingContext() and
LogicalConfirmReceivedLocation().
What I observed in the test was that a walsender process called:
SnapBuildProcessRunningXacts()
LogicalIncreaseXminForSlot()
LogicalConfirmReceivedLocation()
ReplicationSlotsComputeRequiredXmin(false).
In ReplicationSlotsComputeRequiredXmin() it acquired the
ReplicationSlotControlLock and got 0 as the minimum xmin since there
was no wal sender having effective_xmin. Before calling
ProcArraySetReplicationSlotXmin() (i.e. before acquiring
ProcArrayLock), another walsender process called
CreateInitDecodingContext(), acquired ProcArrayLock, computed
slot->effective_catalog_xmin, called
ReplicationSlotsComputeRequiredXmin(true). Since its
effective_catalog_xmin had been set, it got 39968 as the minimum xmin,
and updated replication_slot_xmin. However, as soon as the second
walsender released ProcArrayLock, the first walsender updated the
replication_slot_xmin to 0. After that, the second walsender called
SnapBuildInitialSnapshot(), and GetOldestSafeDecodingTransactionId()
returned an XID newer than snap->xmin.
One idea to fix this issue is that in
ReplicationSlotsComputeRequiredXmin(), we compute the minimum xmin
while holding both ProcArrayLock and ReplicationSlotControlLock, and
release only ReplicationSlotsControlLock before updating the
replication_slot_xmin. I'm concerned it will increase the contention
on ProcArrayLock but I've attached the patch for discussion.
Regards,
[1]
https://www.postgresql.org/message-id/tencent_7EB71DA5D7BA00EB0B429DCE45D0452B6406%40qq.com
--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com
fix_slot_xmin_race_condition.patch
Description: Binary data
