Horton

Re: make MaxBackends available in _PG_init

Anton A. Melnikov <aamelnikov@inbox.ru>

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From: "Anton A. Melnikov" <aamelnikov@inbox.ru>
To: Nathan Bossart <nathandbossart@gmail.com>, 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: 2022-05-06T06:35:05Z
Lists: pgsql-hackers 
Hello!

On 19.04.2022 18:46, Nathan Bossart wrote:
> Okay, I did it this way in v5.
> 

Recently i ran into a problem that it would be preferable in our 
extended pg_stat_statements to use MaxBackEnds in _PG_Init() but it's 
equal to zero here.
And i was very happy to find this patch and this thread.  It was not 
only very interesting and informative for me but also solves the current 
problem. Patch is applied cleanly on current master. Tests under Linux 
and Windows were successful.
I've tried this patch with our extended pg_stat_statements and checked 
that MaxBackends has the correct value during extra shmem allocating. 
Thanks a lot!

+1 for this patch.

I have a little doubt about the comment in postmaster.c: "Now that 
loadable modules have had their chance to alter any GUCs". Perhaps this 
comment is too general. Not sure that it is possible to change any 
arbitrary GUC here.
And maybe not bad to add Assert(size > 0) in RequestAddinShmemSpace() to 
see that the size calculation in contrib was wrong.


With best regards,

-- 
Anton A. Melnikov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

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