Re: make MaxBackends available in _PG_init

On Wed, Apr 13, 2022 at 12:05:08PM -0400, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> It may be too much to hope that we're going to completely get rid of
>> process_shared_preload_libraries_in_progress tests.
>
> Perhaps, but this is definitely an area that could stand to have some
> serious design thought put into it. You're quite right that it's
> largely a mess today, but I think proposals like "forbid setting GUCs
> during _PG_init" would add to the mess rather than clean things up.

+1. The simplest design might be to just make a separate preload hook.
_PG_init() would install hooks, and then this preload hook would do
anything that needs to happen when the library is loaded via s_p_l.
However, I think we still want a new shmem request hook where MaxBackends
is initialized.

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).

--
Nathan Bossart
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