Re: Race between prepared transaction commit and checkpointer

On 9/11/26 11:33, Andrey Borodin wrote:
> Hi Maksim,
>
> On 11 Sep 2026, Maksim Melnikov wrote:
>> I am not sure but for example
>> it can be LookupGXact.
> Yes, your suspicion about LookupGXact() is correct. With your patch
> applied, I could still trigger its Assert(gxact->prepare_start_lsn) by
> pausing in the WAL branch and completing a checkpoint. Both holders
> use LW_SHARED, so the checkpoint can proceed. This was a direct call
> to the helper via a test wrapper, not an end-to-end logical replication
> test.
>
> I also reproduced the original race for both COMMIT PREPARED and
> ROLLBACK PREPARED. Your fix blocks that interleaving in both cases.
>
> Would it be better to take LW_EXCLUSIVE in CheckPointTwoPhase() and
> protect the state reads in both FinishPreparedTransaction() and
> LookupGXact() with LW_SHARED? That would address both readers without
> serializing their I/O under a global exclusive lock. The reader lock
> would still need to cover the WAL read, not just copying the LSN,
> because the checkpoint can otherwise proceed to recycling that WAL.
>
> Thank you!
>
>
> Best regards, Andrey Borodin.
>
>
>
Hi Andrey,

Yes, I agree that we should acquire LW_EXCLUSIVE in CheckPointTwoPhase,
also I agree that better to wrap I/O on FinishPreparedTransaction with
just LW_SHARED, but I have some doubts about LookupGXact bacause as I
see here we change gxact state and it seems better do it with EXCLUSIVE
lock. Maybe we should acquire LW_EXCLUSIVE in LookupGXact and release it
and then acquire LW_SHARED for I/O. What do you think? For you
convenience I've attached draft patch.

Thanks you.

Best regards

Maksim Melnikov