Horton

Re: make MaxBackends available in _PG_init

Julien Rouhaud <rjuju123@gmail.com>

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From: Julien Rouhaud <rjuju123@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>, "Bossart, Nathan" <bossartn@amazon.com>, Fujii Masao <masao.fujii@oss.nttdata.com>, "wangsh.fnst@fujitsu.com" <wangsh.fnst@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Greg Sabino Mullane <htamfids@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-19T09:49:13Z
Lists: pgsql-hackers 
Hi,

On Mon, Apr 18, 2022 at 08:17:04PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
> > I'm looking for a clean way to ERROR if someone attempts to call
> > RequestAddinShmemSpace() or RequestNamedLWLockTranche() outside of the
> > hook.  Currently, we are using static variables in ipci.c and lwlock.c to
> > silently ignore invalid requests.  I could add a new 'extern bool' called
> > 'process_shmem_requests_in_progress', but extensions could easily hack
> > around that to allow requests in _PG_init().  Maybe I am overthinking all
> > this and that is good enough.
> 
> If they do that and it breaks something, that's their fault not ours.
> (It's not like there's not $BIGNUM ways for a C-language module to
> break the backend, anyway.)

Agreed.  Similarly the process_shared_preload_libraries_in_progress flag could
be modified by extension, and that wouldn't be any better.

> BTW, I'd make such errors FATAL, as it's unlikely that we can recover
> cleanly from an error during initialization of a loadable module.
> The module's likely to be only partially initialized/hooked in.

While at it, should we make process_shmem_requests_in_progress true when the
new hook is called?  The hook should only be called when that's the case, and
extension authors may feel like asserting it.

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