Re: make MaxBackends available in _PG_init

On Wed, Apr 13, 2022 at 11:30:40AM -0700, Nathan Bossart wrote:
> 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.

Yeah this one seems needed no matter what.

> 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.