Horton

Re: make MaxBackends available in _PG_init

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Julien Rouhaud <rjuju123@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, 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: 2022-04-14T16:32:20Z
Lists: pgsql-hackers 
On Thu, Apr 14, 2022 at 12:22 PM Nathan Bossart
<nathandbossart@gmail.com> wrote:
> > 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.

I tend to think it's a good idea. It's not great to inconvenience
extension authors, but I think that with the kind of design we are
talking about here it will be pretty straightforward to see how to
update your extension: move the code that request shmem resources into
the new hook. If you want to be compatible with older PostgreSQL
releases by conditional compilation, conditionally call that function
from _PG_init() when PG_VERSION_NUM < whatever.

Compare that with the current situation, where things seem to mostly
work but sometimes don't, and it's not quite clear what to do about
it.

-- 
Robert Haas
EDB: http://www.enterprisedb.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