Possible premature SNAPBUILD_CONSISTENT with DB-specific running_xacts

SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>

From: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>

To: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Álvaro Herrera <alvherre@kurilemu.de>, Antonin Houska <ah@cybertec.at>

Date: 2026-04-19T18:58:46Z

Lists: pgsql-hackers

Attachments

v1-snapbuild-only.patch (application/octet-stream) patch v1

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