Horton

Re: Improve LWLock tranche name visibility across backends

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Sami Imseih <samimseih@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-11-10T18:05:14Z
Lists: pgsql-hackers

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On Mon, Nov 10, 2025 at 10:49:58AM -0600, Nathan Bossart wrote:
> On Mon, Nov 10, 2025 at 10:26:17AM -0600, Nathan Bossart wrote:
>> It's probably a good idea to avoid tranche leaks, but IMHO there's room for
>> improvement in the DSM registry, too.  IIUC the problem is that the DSM
>> segment is still being added to the registry and found by other backends
>> despite the initialization callback failing.  My first instinct is that we
>> should keep track of whether the DSM segments/DSAs/dshash tables in the
>> registry have been fully initialized and to just ERROR in other backends
>> when attaching if they aren't.  That shouldn't really happen in practice,
>> but it'd be good to avoid the strange errors, anyway.

0001 does this.  When applied, Alexander's reproduction steps fail with

	ERROR:  requested DSM segment failed initialization

This one should probably get back-patched to v17, where the DSM registry
was introduced.

> BTW if we really wanted to avoid leaking tranches in test_dsa, we'd need to
> store the ID in shared memory.  Your patch helps in the case where a single
> backend is repeatedly calling test_dsa_resowners(), but other backends
> would still allocate their own tranche.  I don't see a strong need to fix
> this on back-branches, given these functions run exactly once as part of a
> test, but fixing it on master seems worthwhile so that extension authors
> don't copy/paste this broken code.

0002 does this.

-- 
nathan

Commits

  1. test_dsa: Avoid leaking LWLock tranches.

  2. Teach DSM registry to ERROR if attaching to an uninitialized entry.

  3. Add a test harness for the LWLock tranche code.

  4. Revert recent change to RequestNamedLWLockTranche().

  5. Move dynamically-allocated LWLock tranche names to shared memory.

  6. Prepare DSM registry for upcoming changes to LWLock tranche names.

  7. Add GetNamedDSA() and GetNamedDSHash().