Reserving GUC prefixes from a non-preloaded DB extension is not always enforced

Hi all,

I am an extension developer. I use `MarkGUCPrefixReserved` to reserve GUC
prefixes, which my extension uses to help avoid accidentally misspelled
config-file entries.

However, since the reservation happens in `_PG_init()` and `_PG_init()` is
not called until the first use of an API exposed by my extension,
misspelled config-file entries that get executed before the extension is
loaded will not throw an error.

SET lantern.haha = 1; -- succeeds, since lantern extension is not loaded

SELECT ARRAY[1] <-> ARRAY[1]; -- uses a lantern API, so extension binary is
loaded
-- The line above does warn about removing the configuration parameter above
-- WARNING: invalid configuration parameter name "lantern.haha", removing
it
-- DETAIL: "lantern" is now a reserved prefix.

SET lantern.haha = 1; -- now this throws an error
-- ERROR: invalid configuration parameter name "lantern.haha"
-- DETAIL: "lantern" is a reserved prefix.

I think, ideally, the last error should be thrown in the first SET
execution as well.

I'd expect GUC variables reserved by an extension to live more permanently
in Postgres catalogs (e.g., in pg_settings).
So, even when the extension binary is not loaded, Postgres would know which
prefixes are reserved and which GUC settings must be allowed (similar to
how Postgres knows in pg_extension which extensions are enabled, even when
the corresponding extension binary has not been loaded).

1. Would you consider the proposed behavior an improvement?

2. If so, do you have thoughts on how to implement it?

Thanks!
Narek Galstyan
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