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: Rahila Syed <rahilasyed90@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-12T19:48:50Z
Lists: pgsql-hackers 
I haven't followed the latest discussion, but I took a look at the patch.

+      It is possible to use a <literal>tranche_id</literal> that was not retrieved
+      using <function>LWLockNewTrancheId</function>, but this is not recommended.
+      The ID may clash with an already registered tranche name, or the specified
+      name may not be found. In such cases, looking up the name will return a generic
+      "extension" tranche name.

Is there any reason to continue allowing this?  For example, maybe we could
ERROR if LWLockInitialize()/GetLWTrancheName() are given a tranche_id
greater than the number allocated.  I guess I'm not following why we should
gracefully handle these kinds of coding errors, especially when they result
in unhelpful behavior like an "extension" tranche.

+#else
+elog(ERROR, "injection points not supported");
+#endif

This causes compilation to fail when injection points are not enabled.

I haven't combed through the patch character-by-character, but upon a
read-through, the general shape looks reasonable to me.  As a general note,
I'd suggest adding more commentary throughout and finding opportunities to
simplify and/or clean up the code as much as possible.

-- 
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().