Horton

Re: Improve LWLock tranche name visibility across backends

Sami Imseih <samimseih@gmail.com>

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From: Sami Imseih <samimseih@gmail.com>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-07-14T20:45:02Z
Lists: pgsql-hackers 
> Ah, I missed the problem with postmaster.  Could we have the first backend
> that needs to access the table be responsible for creating it and
> populating it with the built-in/requested-at-startup entries?

We can certainly maintain a flag in the shared state that is set once
the first backend loads all the tranches in shared memory. That did not
cross my mind, but it feels wrong to offload such responsibility to a
normal backend.

> Also, is there any chance that postmaster might need to access the
> tranche names?

A postmaster does not currently have a reason to lookup
a tranche name, afaict. This only occurs when looking up wait events
or if lwlock tracing is enabled.

--
Sami

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().