Re: make MaxBackends available in _PG_init

On Tue, Mar 29, 2022 at 12:22:21PM -0400, Robert Haas wrote:
> 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.

FWIW I would be on board with reverting all the GetMaxBackends() stuff if
we made the value available in _PG_init() and stopped supporting GUC
overrides by extensions (e.g., ERROR-ing in SetConfigOption() when
process_shared_preload_libraries_in_progress is true).

--
Nathan Bossart
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