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From: Sami Imseih <[email protected]>
To: Heikki Linnakangas <[email protected]>
Cc: Matthias van de Meent <[email protected]>
Cc: Nathan Bossart <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Duplicate RequestNamedLWLocktranche() names and test_lwlock_tranches improvements
Date: Sun, 5 Apr 2026 11:05:14 -0500
Message-ID: <CAA5RZ0u+=t0HO_wBWnw-fB=AAoriyHTO0=rH3T+GFgsLM1Ukqw@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

> Starting new thread for this thing that Matthias noticed in my
> work-in-progress patch at
>
> https://www.postgresql.org/message-id/[email protected]...
> .
>
> On 05/04/2026 02:17, Matthias van de Meent wrote:
> > 0006: I don't think it is a great idea to make the LwLock machinery
> > the first to get allocation requests:
> > It has the RequestNamedLWLockTranche infrastructure, which can only
> > register new requests while process_shmem_requests_in_progress, and
> > making it request its memory ahead of everything else is likely to
> > cause an undersized tranche to be allocated. You could make sure that
> > this isn't an issue by maintaining a flag in lwlock.c that's set when
> > the shmem request is made (and reset on shmem exit), which must be
> > false when RequestNamedLWLockTranche() is called, and if not then it
> > should throw an error.
>
> Good catch, RequestNamedLWLocktranche() was quite broken with the patch.
> I'm surprised it didn't cause test failures. We even have unit tests for
> that at src/test/modules/test_lwlock_tranches.
>
> Looking at src/test/modules/test_lwlock_tranches, I realized that we
> don't currently check that the tranche name registered with
> RequestNamedLWLocktranche() is unique. If two extensions registered a
> tranche with same name, we'd allocate two separate tranches for them,
> but GetNamedLWLockTranche() would always return the first one.


Yes, while that is not very likely scenario, it’s wrong. The caller will
get the wrong pointer to the locks.

>
>
> Attached patches add a uniqueness check, and improves
> test_lwlock_tranches so that it actually uses the requested LWLocks. And
> I couldn't resist doing some more refactoring of the test while I was at
> it; IMO it's more readable now.
>
> Barring objections, I will commit these shortly.
>

LGTM

--
Sami Imseih
Amazon Web Services (AWS)

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  Subject: Re: Duplicate RequestNamedLWLocktranche() names and test_lwlock_tranches improvements
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