MIME-Version: 1.0
From: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Date: Sun, 19 Apr 2026 11:58:46 -0700
Message-ID: 
 <CAHg+QDcQak4jx_6X2_Ws98rzG=xBARLjqm_=56wTRUtNsY4DZQ@mail.gmail.com>
Subject: Possible premature SNAPBUILD_CONSISTENT with DB-specific
 running_xacts
To: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	=?UTF-8?Q?=C3=81lvaro_Herrera?= <alvherre@kurilemu.de>,
	Antonin Houska <ah@cybertec.at>
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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

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<div dir=3D"ltr">Hi hackers,<div><br></div><div>A cluster-wide decoder must=
 never have its snapshot-builder state changed<br>by a database-specific ru=
nning_xacts record. Adding a check to return it early.</div><div>I think ot=
herwise a cluster wide decoder can potentially go to=C2=A0</div><div>SNAPSH=
OT_CONSISTENT state immediately even though transactions older</div><div>th=
an nextXid are still in progress on a different DB (not tracked by running_=
xact</div><div>record). This race is now possible with cluster wide decoder=
s and Repack</div><div>concurrently run.</div><div><br></div><div><br></div=
><div>Attached a patch to fix this. Thoughts?</div><div><br></div><div>Than=
ks</div><div>Satya</div></div>

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