Date: Mon, 25 Aug 2025 13:31:26 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Sami Imseih <samimseih@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: GetNamedLWLockTranche crashes on Windows in normal backend
Message-ID: <aKyr_rVOtlolDcy-@nathan>
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On Mon, Aug 25, 2025 at 12:58:08PM -0500, Sami Imseih wrote:
>> If this fails, why doesn't the call in pgss_shmem_startup() also fail?  Was
>> pg_stat_statements not loaded via shared_preload_libraries?
> 
> because the array is valid in postmaster, but it's not for a normal backend,
> since it's not getting copied.
> 
> Yes, pg_stat_statements was loaded via shared_preload_libraries. Nothing
> was done out of the ordinary.
> 
>> And is the
>> failure a segfault or a "requested tranche is not registered" error?
> 
> It's a segfault.

Oh, I see what's happening.  We are never calling GetNamedLWLockTranche()
in backends because ShmemInitStruct() always sets *found.

I'd argue that calling GetNamedLWLockTranche() outside of an "if (!found)"
block in a shmem_startup_hook doesn't follow convention, but the docs have
this note:

	shmem_startup_hook provides a convenient place for the initialization
	code, but it is not strictly required that all such code be placed in
	this hook.

Like the inaccurate sentence immediately following this one [0], it was
added by commit 964152c.  IMHO we should adjust the documentation to match
reality.

[0] https://postgr.es/m/aJ9QIF3g2QQko9wq%40nathan

-- 
nathan