From: Michael Paquier <michael@paquier.xyz>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Fix rare instability in recovery TAP test 009_twophase
Date: Wed, 04 Mar 2026 07:32:11 +0000
Message-ID: <E1vxgiJ-002J9X-13@gemulon.postgresql.org> (raw)
Fix rare instability in recovery TAP test 009_twophase
The phase of the test where we want to check that 2PC transactions
prepared on a primary can be committed on a promoted standby relied on
an immediate stop of the primary. This logic has a race condition: it
could be possible that some records (most likely standby snapshot
records) are generated on the primary before it finishes its shutdown,
without the promoted standby know about them. When the primary is
recycled as new standby, the test could fail because of a timeline fork
as an effect of these extra records.
This fix takes care of the instability by doing a clean stop of the
primary instead of a teardown (aka immediate stop), so as all records
generated on the primary are sent to the promoted standby and flushed
there. There is no need for a teardown of the primary in this test
scenario: the commit of 2PC transactions on a promoted standby do not
care about the state of the primary, only of the standby.
This race is very hard to hit in practice, even slow buildfarm members
like skink have a very low rate of reproduction. Alexander Lakhin has
come up with a recipe to improve the reproduction rate a lot:
- Enable -DWAL_DEBUG.
- Patch the bgwriter so as standby snapshots are generated every
milliseconds.
- Run 009_twophase tests under heavy parallelism.
With this method, the failure appears after a couple of iterations.
With the fix in place, I have been able to run more than 50 iterations
of the parallel test sequence, without seeing a failure.
Issue introduced in 30820982b295, due to a copy-pasto coming from the
surrounding tests. Thanks also to Hayato Kuroda for digging into the
details of the failure. He has proposed a fix different than the one of
this commit. Unfortunately, it relied on injection points, feature only
available in v17. The solution of this commit is simpler, and can be
applied to v14~v16.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/b0102688-6d6c-c86a-db79-e0e91d245b1a@gmail.com
Backpatch-through: 14
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/ad8c86d22cbd043cfb48cc3c18e43c4f267b2eb0
Modified Files
--------------
src/test/recovery/t/009_twophase.pl | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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To: pgsql-committers@postgresql.org
Cc: michael@paquier.xyz, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Fix rare instability in recovery TAP test 009_twophase
In-Reply-To: <E1vxgiJ-002J9X-13@gemulon.postgresql.org>
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