Re: Assertion failure in SnapBuildInitialSnapshot()

On Mon, Nov 21, 2022 at 4:31 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Sat, Nov 19, 2022 at 6:35 AM Andres Freund <andres(at)anarazel(dot)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

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Masahiko Sawada
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