Horton

Re: make MaxBackends available in _PG_init

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Julien Rouhaud <rjuju123@gmail.com>, 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: 2022-04-12T19:21:57Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> Gah, I hate putting this off for another year, but I guess I'm also
> not convinced that 0003 has the right idea, so maybe it's for the
> best. Here's what's bugging me: 0003 decrees that _PG_init() is the
> Wrong Place to Adjust GUC Settings. Now, that begs the question: what
> is the right place to adjust GUC settings? I argued upthread that
> extensions shouldn't be overriding values from postgresql.conf at all,
> but on further reflection I think there's at least one legitimate use
> case for this sort of thing: some kind of auto-tuning module that, for
> example, looks at how much memory you have and sets shared_buffers or
> work_mem or something based on the result.

Yeah.  It's a very long way from "changing shared memory sizes here
doesn't work" to "no extension is allowed to change any GUC at load
time".  A counterexample is that it's not clear why an extension
shouldn't be allowed to define a GUC using DefineCustomXXXVariable
and then immediately set it to some other value.  I think trying to
forbid that across-the-board is going to mostly result in breaking
a lot of cases that are perfectly safe.

			regards, tom lane

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