Horton

Re: make MaxBackends available in _PG_init

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: "Bossart, Nathan" <bossartn@amazon.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2021-08-02T20:37:24Z
Lists: pgsql-hackers 
Hi,

On 2021-08-02 16:06:15 -0400, Robert Haas wrote:
> On Mon, Aug 2, 2021 at 2:18 PM Bossart, Nathan <bossartn@amazon.com> wrote:
> > I just encountered the same thing, so I am bumping this thread.  I was
> > trying to use MaxBackends in a call to RequestAddinShmemSpace() in a
> > _PG_init() function for a module, but since MaxBackends is not yet
> > initialized, you essentially need to open-code InitializeMaxBackends()
> > instead.
> >
> > I think the comments about needing to register background workers
> > before initializing MaxBackends have been incorrect since the addition
> > of max_worker_processes in v9.4 (6bc8ef0b).  Furthermore, I think the
> > suggested reordering is a good idea because it is not obvious that
> > MaxBackends will be uninitialized in _PG_init(), and use-cases like
> > the RequestAddinShmemSpace() one are not guaranteed to fail when
> > MaxBackends is used incorrectly (presumably due to the 100 KB buffer
> > added in CreateSharedMemoryAndSemaphores()).
> >
> > I've attached a new version of the proposed patch with some slight
> > adjustments and an attempt at a commit message.
> 
> I think this is a good idea. Anyone object?

I'm not so sure it's a good idea. I've seen several shared_preload_library
using extensions that adjust some GUCs (e.g. max_prepared_transactions)
because they need some more resources internally - that's perhaps not a great
idea, but there's also not an obviously better way.

ISTM this would better be solved with making the hook or config logic for
libraries a bit more elaborate (e.g. having a hook you can register for that's
called once all libraries are loaded).

Greetings,

Andres Freund

Commits

  1. Add a new shmem_request_hook hook.

  2. Remove non-functional code for unloading loadable modules.

  3. Fix misleading comments about background worker registration.

  4. Revert the addition of GetMaxBackends() and related stuff.

  5. Fix typo in multixact.c

  6. Reduce more the number of calls to GetMaxBackends()

  7. Remove MaxBackends variable in favor of GetMaxBackends() function.

  8. Fix comments about bgworker registration before MaxBackends initialization

  9. Fix bogus assertion in BootstrapModeMain().

  10. Make sure MaxBackends is always set