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Commits

  1. Check that the tranche name is unique in RequestNamedLWLockTranche

  2. Improve test_lwlock_tranches

  1. Duplicate RequestNamedLWLocktranche() names and test_lwlock_tranches improvements

    Heikki Linnakangas <hlinnaka@iki.fi> — 2026-04-05T14:28:53Z

    Starting new thread for this thing that Matthias noticed in my 
work-in-progress patch at 
https://www.postgresql.org/message-id/CAEze2WjgCROMMXY0+j8FFdm3iFcr7By-+6Mwiz=PgGSEydiW3A@mail.gmail.com.

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.

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.

- Heikki

  2. Re: Duplicate RequestNamedLWLocktranche() names and test_lwlock_tranches improvements

    Sami Imseih <samimseih@gmail.com> — 2026-04-05T16:05:14Z

    > Starting new thread for this thing that Matthias noticed in my
> work-in-progress patch at
>
> https://www.postgresql.org/message-id/CAEze2WjgCROMMXY0+j8FFdm3iFcr7By-+6Mwiz=PgGSEydiW3A@mail.gmail.com
> .
>
> 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)

  3. Re: Duplicate RequestNamedLWLocktranche() names and test_lwlock_tranches improvements

    Heikki Linnakangas <hlinnaka@iki.fi> — 2026-04-05T18:24:35Z

    On 05/04/2026 19:05, Sami Imseih wrote:
> LGTM

Committed, thanks!

- Heikki