Horton
Menu

Thread

  1. Possible premature SNAPBUILD_CONSISTENT with DB-specific running_xacts

    SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com> — 2026-04-19T18:58:46Z

    Hi hackers,

A cluster-wide decoder must never have its snapshot-builder state changed
by a database-specific running_xacts record. Adding a check to return it
early.
I think otherwise a cluster wide decoder can potentially go to
SNAPSHOT_CONSISTENT state immediately even though transactions older
than nextXid are still in progress on a different DB (not tracked by
running_xact
record). This race is now possible with cluster wide decoders and Repack
concurrently run.


Attached a patch to fix this. Thoughts?

Thanks
Satya

  2. Re: Possible premature SNAPBUILD_CONSISTENT with DB-specific running_xacts

    Amit Kapila <amit.kapila16@gmail.com> — 2026-04-20T06:04:17Z

    On Mon, Apr 20, 2026 at 12:29 AM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> A cluster-wide decoder must never have its snapshot-builder state changed
> by a database-specific running_xacts record. Adding a check to return it early.
> I think otherwise a cluster wide decoder can potentially go to
> SNAPSHOT_CONSISTENT state immediately even though transactions older
> than nextXid are still in progress on a different DB (not tracked by running_xact
> record). This race is now possible with cluster wide decoders and Repack
> concurrently run.
>

I think this has been discussed previously, see [1]. As per my
understanding, we are not in agreement for the need of this
db-specific handling in the snapbuilder, see [2][3]. So, adding more
improvements/fixes on top of it doesn't sound advisable.

[1] - https://www.postgresql.org/message-id/CAA4eK1KWDbBk4FgbbWdivQLrPPzR4zgvfnHK4WjWC78rbuRVbg%40mail.gmail.com
[2] - https://www.postgresql.org/message-id/cdgw4sbbfcgk6du3iv54r2dgiy4tfywoklbotlmj4irxavdcr3%40glxfw5jj277q
[3] - https://www.postgresql.org/message-id/pveffyxhnuurhb44uzqlwo3rkyzorkfh2rot7uwzlf2axhfvbp%407nrs2omysxkc

-- 
With Regards,
Amit Kapila.