Re: make MaxBackends available in _PG_init

On Wed, Apr 13, 2022 at 11:39 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Is there anything else typically done in _PG_init that has to be
> conditional on process_shared_preload_libraries_in_progress? I recall
> something about reserving LWLocks, which probably should get the same
> treatment. If we can get to a point where _PG_init shouldn't need to
> care about process_shared_preload_libraries_in_progress, because all
> the stuff that would care is moved to this new hook, then that would
> be very clear cleanup.

What's a little wonky right now is that it's fairly common for
extensions to just return straight-off without doing *anything at all*
if !process_shared_preload_libraries_in_progress. See
pg_stat_statements for example. It seems kind of strange to me to make
registering GUCs dependent on
process_shared_preload_libraries_in_progress; why shouldn't that just
happen always? It's understandable that we don't want to install the
hook functions if we're not being loaded from shared_preload_libaries,
though.

It may be too much to hope that we're going to completely get rid of
process_shared_preload_libraries_in_progress tests. But even given
that, I think having a request-shared-mem hook makes sense and would
make things cleaner than they are today. Maybe in the future we'll end
up with other hooks as well, like a "this is the place to register
GUCs" hook and a perhaps a "this is the place to register you custom
rmgr" hook. I'm not really sure that we need those and I don't want to
make things complicated just for the heck of it, but it's not shocking
that different operations need to happen at different times in the
startup sequence, and I don't think we should be afraid to split up
the monolithic _PG_init() into as many separate things as are required
to make it work right.

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