Re: warn if GUC set to an invalid shared library

On Fri, Jul 22, 2022 at 01:53:21PM -0400, Tom Lane wrote:
> > 2022-07-22 10:37:50.217 PDT [1131187] LOG:  database system is shut down
> > 2022-07-22 10:37:50.306 PDT [1134058] WARNING:  could not access file "$libdir/plugins/lol"
> > 2022-07-22 10:37:50.306 PDT [1134058] DETAIL:  The server will currently fail to start with this setting.
> > 2022-07-22 10:37:50.306 PDT [1134058] HINT:  If the server is shut down, it will be necessary to manually edit the postgresql.auto.conf file to allow it to start again.
> > 2022-07-22 10:37:50.312 PDT [1134058] FATAL:  could not access file "lol": No such file or directory
> > 2022-07-22 10:37:50.312 PDT [1134058] CONTEXT:  while loading shared libraries for setting "shared_preload_libraries" from /home/maciek/code/aux/postgres/tmpdb/postgresql.auto.conf:3
> > 2022-07-22 10:37:50.312 PDT [1134058] LOG:  database system is shut down
> 
> > Hmm, I guess this is a side effect of where these messages are
> > emitted, but at this point, lines 4-6 here are a little confusing, no?
> 
> This indicates that the warning is being issued in the wrong place.
> It's okay if it comes out during ALTER SYSTEM.  It's not okay if it
> comes out during server start; then it's just an annoyance.

This was a regression from the previous patch version, and I even noticed the
problem, but then forgot when returning to the patch :(

The previous patch version checked if (!IsUnderPostmaster()) before warning.
Is there a better way ?

ALTER SYSTEM uses PGC_S_FILE, the same as during startup..

-- 
Justin