Horton

Re: make MaxBackends available in _PG_init

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Julien Rouhaud <rjuju123@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, 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: 2022-04-14T16:22:15Z
Lists: pgsql-hackers 
On Thu, Apr 14, 2022 at 01:50:24PM +0800, Julien Rouhaud wrote:
> On Wed, Apr 13, 2022 at 11:30:40AM -0700, Nathan Bossart wrote:
>> If we do move forward with the shmem request hook, do we want to disallow
>> shmem requests anywhere else, or should we just leave it up to extension
>> authors to do the right thing?  Many shmem requests in _PG_init() are
>> probably okay unless they depend on MaxBackends or another GUC that someone
>> might change.  Given that, I think I currently prefer the latter (option B
>> from upthread).
> 
> I'd be in favor of a hard break.  There are already multiple extensions that
> relies on non final value of GUCs to size their shmem request.  And as an
> extension author it can be hard to realize that, as those extensions work just
> fine until someone wants to try it with some other extension that changes some
> GUC.  Forcing shmem request in a new hook will make sure that it's *always*
> correct, and that only requires very minimal work on the extension side.

Yeah, this is a good point.  If we're okay with breaking existing
extensions like this, I will work on a patch.

-- 
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