Horton

Re: make MaxBackends available in _PG_init

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Julien Rouhaud <rjuju123@gmail.com>
Cc: Nathan Bossart <nathandbossart@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>, Tom Lane <tgl@sss.pgh.pa.us>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-11T14:47:17Z
Lists: pgsql-hackers 
On Sat, Apr 9, 2022 at 9:24 AM Julien Rouhaud <rjuju123@gmail.com> wrote:
> > FWIW I would be on board with reverting all the GetMaxBackends() stuff if
> > we made the value available in _PG_init() and stopped supporting GUC
> > overrides by extensions (e.g., ERROR-ing in SetConfigOption() when
> > process_shared_preload_libraries_in_progress is true).
>
> Yeah I would prefer this approach too, although it couldn't prevent extension
> from directly modifying the underlying variables so I don't know how effective
> it would be.

I think I also prefer this approach. I am willing to be convinced
that's the wrong idea, but right now I favor it.

> On the bright side, I see that citus is using SetConfigOption() to increase
> max_prepared_transactions [1].  That's the only extension mentioned in that
> thread that does modify some GUC, and this GUC was already marked as
> PGDLLIMPORT since 2017 so it probably wasn't done that way for Windows
> compatibility reason.

I don't quite understand this, but I think if we want to support this
kind of thing it needs a separate hook.

-- 
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