Re: Possibility to disable `ALTER SYSTEM`

I want to remind everyone of this from Gabriele's first message that
started this thread:

> At the moment, a possible workaround is that `ALTER SYSTEM` can be blocked
> by making the postgresql.auto.conf read only, but the returned message is
> misleading and that’s certainly bad user experience (which is very
> important in a cloud native environment):
>
>
> ```
> postgres=# ALTER SYSTEM SET wal_level TO minimal;
> ERROR:  could not open file "postgresql.auto.conf": Permission denied
> ```

I think making the config file read-only is a fine solution. If you
don't want postgres to mess with the config files, forbid it with the
permission system.

Problems with pg_rewind, pg_basebackup were mentioned with that
approach. I think if you want the config files to be managed outside
PostgreSQL, by kubernetes, patroni or whatever, it would be good for
them to be read-only to the postgres user anyway, even if we had a
mechanism to disable ALTER SYSTEM. So it would be good to fix the
problems with those tools anyway.

The error message is not great, I agree with that. Can we improve it?
Maybe just add a HINT like this:

postgres=# ALTER SYSTEM SET wal_level TO minimal;
ERROR:  could not open file "postgresql.auto.conf" for writing:
Permission denied
HINT:  Configuration might be managed outside PostgreSQL


Perhaps we could make that even better with a GUC though. I propose a
GUC called 'configuration_managed_externally = true / false". If you set
it to true, we prevent ALTER SYSTEM and make the error message more
definitive:

postgres=# ALTER SYSTEM SET wal_level TO minimal;
ERROR:  configuration is managed externally

As a bonus, if that GUC is set, we could even check at server startup
that all the configuration files are not writable by the postgres user,
and print a warning or refuse to start up if they are.

(Another way to read this proposal is to rename the GUC that's been
discussed in this thread to 'configuration_managed_externally'. That
makes it look less like a security feature, and describes the intended
use case.)