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: 2022-04-12T21:07:45Z
Lists: pgsql-hackers 
On Tue, Apr 12, 2022 at 04:58:42PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>> I think it'd be reasonable to allow changing custom GUCs in _PG_init().
>> Those aren't going to impact things like MaxBackends.
> 
> It's reasonable to allow changing *most* GUCs in _PG_init(); let us
> please not break cases that work fine today.

Right, this is a fair point.

> It seems after a bit of reflection that what we ought to do is identify
> the subset of PGC_POSTMASTER GUCs that feed into shared memory sizing,
> mark those with a new GUC flag, and not allow them to be changed after
> shared memory is set up.  (An alternative to a new flag bit is to
> subdivide the PGC_POSTMASTER context into two values, but that seems
> like it'd be far more invasive.  A flag bit needn't be user-visible.)
> Then, what we'd need to do is arrange for shared-preload modules to
> receive their _PG_init call before shared memory creation (when they
> can still twiddle such GUCs) and then another callback later.

If we allow changing GUCs in _PG_init() and provide another hook where
MaxBackends will be initialized, do we need to introduce another GUC flag,
or can we get away with just blocking all GUC changes when the new hook is
called?  I'm slightly hesitant to add a GUC flag that will need to manually
maintained.  Wouldn't it be easily forgotten?

-- 
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