Re: make MaxBackends available in _PG_init

Hi,

On Fri, Mar 25, 2022 at 03:23:17PM -0700, Andres Freund wrote:
>
> I don't really understand. The issue that started this thread was bugs in
> extensions due to accessing MaxBackends before it is initialized - which the
> patch prevents.

Well, the patch prevents accessing a 0-valued MaxBackends but doesn't do
anything to solve the original complaint. It's not like extensions won't need
to access that information during _PG_init anymore.

> The stuff that you're complaining about / designing here
> doesn't seem related to that. I like the idea of the hooks etc, but I fail to
> see why we "ought to revisit the business with GetMaxBackends()"?

Because this GetMaxBackends() doesn't solve the problem nor brings any
infrastructure that could be reused to solve it.

I think that the root issue could be rephrased with:

Can we initialize MaxBackends earlier so that _PG_init() can see it because
maintaining MaxConnections + autovacuum_max_workers + 1 + max_worker_processes
+ max_wal_senders is troublesome.

And indeed, any third party code that previously needed to access what
MaxBackends is supposed to store should already be using that formula, and
the new GetMaxBackends() doesn't do anything about it. So all extensions will
be as broken as before, except the few that were using MaxBackends without
realizing it's 0. And if those exist (there's actually one) they're not that
broken, probably because MaxBackend is only used to request additional shmem,
with wanted value small enough so that it's compensated by the extra 100kB
shmem postgres allocates.

Since all those underlying GUCs shouldn't be accessible, we need some more
general infrastructure that would work for those too on top of a way to access
MaxBackends when extensions needs it.

Note that the only use case I'm aware of is for RequestAddinShmemSpace, so a
hook after _PG_init like the example shmem_request_hook would be enough for the
latter, but maybe there are more use cases for which it wouldn't.