Re: make MaxBackends available in _PG_init

On Sat, Mar 26, 2022 at 1:23 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > 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.
>
> It couldn't rely on MaxBackends before. It can't rely on GetMaxBackends()
> now. You can see why I think that what you want is unrelated to the
> introduction of GetMaxBackends().

It's not, though, because the original proposal was to change things
around so that the value of MaxBackends would have been reliable in
_PG_init(). If we'd done that, then extensions that are using it in
_PG_init() would have gone from being buggy to being not-buggy. But
since you advocated against that change, we instead committed
something that caused them to go from being buggy to failing outright.
That's pretty painful for people with such extensions. And IMHO, it's
*much* more legitimate to want to size a data structure based on the
value of MaxBackends than it is for extensions to override GUC values.
If we can make the latter use case work in a sane way, OK, although I
have my doubts about how sane it really is, but it can't be at the
expense of telling extensions that have been (incorrectly) using
MaxBackends in _PG_init() that we're throwing them under the bus.

IMHO, the proper thing to do if certain GUC values are required for an
extension to work is to put that information in the documentation and
error out at an appropriate point if the user does not follow the
directions. Then this issue does not arise. But there's no reasonable
workaround for being unable to size data structures based on
MaxBackends.

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