Horton

Re: shmem_startup_hook called twice on Windows

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Sami Imseih <samimseih@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-15T15:51:24Z
Lists: pgsql-hackers 
On Fri, Aug 15, 2025 at 10:36:47AM -0500, Sami Imseih wrote:
> But that could potentially be dangerous if code in the startup hook gets
> re-executed?  I guess the doc below is giving a vague warning that one
> should be careful what they put in that hook.

The docs seem reasonably clear that these hooks need to be careful to not
re-initialize shared memory that was already initialized by another backend
[0].

> Thanks, I missed the doc update. Yes, that is inconsistent between platforms,
> and if we must live with this behavior, should the doc give a bigger warning
> about the code that goes in that hook?

The docs should definitely be updated for accuracy, but I'm not following
what sort of additional warning you think we need.  Could you share a
concrete example of what you have in mind?

[0] https://www.postgresql.org/docs/devel/xfunc-c.html#XFUNC-SHARED-ADDIN

-- 
nathan

Commits

  1. Fix documentation for shmem_startup_hook.

  2. test_slru: Fix LWLock tranche allocation in EXEC_BACKEND builds.

  3. doc: Reorganize section for shared memory and LWLocks.

  4. Refactor CreateSharedMemoryAndSemaphores