Re: Add a GUC check hook to ensure summarize_wal cannot be enabled when wal_level is minimal

Nathan Bossart <nathandbossart@gmail.com> writes:
> I haven't tested it, but from skimming around the code, it looks like
> ProcessConfigFileInternal() would deduplicate any previous entries in the
> file prior to applying the values and running the check hooks.  Else,
> reloading a configuration file with multiple startup-only GUC entries could
> fail, even without bogus GUC check hooks.

While it's been a little while since I actually traced the logic,
I believe the reason that case doesn't fail is this bit in
set_config_with_handle, about line 3477 as of HEAD:

        case PGC_POSTMASTER:
            if (context == PGC_SIGHUP)
            {
                /*
                 * We are re-reading a PGC_POSTMASTER variable from
                 * postgresql.conf.  We can't change the setting, so we should
                 * give a warning if the DBA tries to change it.  However,
                 * because of variant formats, canonicalization by check
                 * hooks, etc, we can't just compare the given string directly
                 * to what's stored.  Set a flag to check below after we have
                 * the final storable value.
                 */
                prohibitValueChange = true;
            }
            else if (context != PGC_POSTMASTER)
                // throw "cannot be changed now" error

			regards, tom lane