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

Nathan Bossart <nathandbossart(at)gmail(dot)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