Horton

Re: make MaxBackends available in _PG_init

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, 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: 2022-04-12T19:59:08Z
Lists: pgsql-hackers 
On Tue, Apr 12, 2022 at 03:30:23PM -0400, Robert Haas wrote:
> On Tue, Apr 12, 2022 at 3:22 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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.
> 
> Hmm, that's an interesting case which I hadn't considered. It seems
> like sort of a lame thing to do, because why wouldn't you just create
> the GUC with the right initial value instead of modifying it after the
> fact? But maybe there's some use case in which it makes sense to do it
> that way. A read-only GUC that advertises some calculated value,
> perhaps?

I think it'd be reasonable to allow changing custom GUCs in _PG_init().
Those aren't going to impact things like MaxBackends.

-- 
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