Horton

Re: make MaxBackends available in _PG_init

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@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: 2022-04-12T21:01:12Z
Lists: pgsql-hackers 
On Tue, Apr 12, 2022 at 04:33:39PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>> On Tue, Apr 12, 2022 at 03:12:42PM -0400, Robert Haas wrote:
>>> But if there's even one use case where adjusting GUCs at this phase is
>>> reasonable, then 0003 isn't really good enough. We need an 0004 that
>>> provides a new hook in a place where such changes can safely be made.
> 
>> I think that is doable.  IMO it should be ѕomething like _PG_change_GUCs()
>> that is called before _PG_init().  The other option is to add a hook called
>> after _PG_init() where MaxBackends is available (in which case we likely
>> want GetMaxBackends() again).  Thoughts?
> 
> I like the second option.  Calling into a module before we've called its
> _PG_init function is just weird, and will cause no end of confusion.

Alright.  I think that is basically what Julien recommended a few weeks ago
[0].  Besides possibly reintroducing GetMaxBackends(), we might also want
to block SetConfigOption() when called in this new hook.

[0] https://postgr.es/m/20220325031146.cd23gaak5qlzdy6l%40jrouhaud

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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