Re: make MaxBackends available in _PG_init

Hi,

On Mon, Apr 11, 2022 at 02:14:35PM -0700, Nathan Bossart wrote:
> On Mon, Apr 11, 2022 at 01:44:42PM -0700, Nathan Bossart wrote:
> > On Mon, Apr 11, 2022 at 04:36:36PM -0400, Robert Haas wrote:
> >> If we throw an error while defining_custom_guc is true, how will it
> >> ever again become false?
> >
> > Ah, I knew I was forgetting something this morning.
> >
> > It won't, but the only place it is presently needed is when loading
> > shared_preload_libraries, so I believe startup will fail anyway. However,
> > I can see defining_custom_guc being used elsewhere, so that is probably not
> > good enough. Another approach could be to add a static
> > set_config_option_internal() function with a boolean argument to indicate
> > whether it is being used while defining a custom GUC. I'll adjust 0003
> > with that approach unless a better idea prevails.
>
> Here's a new patch set. I've only changed 0003 as described above. My
> apologies for the unnecessary round trip.

It looks sensible to me, although I think I would apply 0003 before 0002.

+ if (process_shared_preload_libraries_in_progress &&
+ !allow_when_loading_preload_libs)
+ ereport(ERROR,
+ (errcode(ERRCODE_CANT_CHANGE_RUNTIME_PARAM),
+ errmsg("cannot change parameters while loading "
+ "\"shared_preload_libraries\"")));
+

I think the error message should mention at least which GUC is being modified.