Re: Two small bugs in guc.c

On Tue Dec 26, 2023 at 1:02 PM CST, Tom Lane wrote:
> I investigated the report at [1] about pg_file_settings not reporting
> invalid values of "log_connections".  It turns out it's broken for
> PGC_BACKEND and PGC_SU_BACKEND parameters, but not other ones.
> The cause is a bit of premature optimization in this logic:
>
>                  * If a PGC_BACKEND or PGC_SU_BACKEND parameter is changed in
>                  * the config file, we want to accept the new value in the
>                  * postmaster (whence it will propagate to
>                  * subsequently-started backends), but ignore it in existing
>                  * backends. ...
>
> Upon detecting that case, set_config_option just returns -1 immediately
> without bothering to validate the value.  It should check for invalid
> input before returning -1, which we can mechanize with a one-line fix:
>
> -                    return -1;
> +                    changeVal = false;
>
> While studying this, I also noted that the bit to prevent changes in
> parallel workers seems seriously broken:
>
>     if (IsInParallelMode() && changeVal && action != GUC_ACTION_SAVE)
>         ereport(elevel,
>                 (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
>                  errmsg("cannot set parameters during a parallel operation")));
>
> This is evidently assuming that ereport() won't return, which seems
> like a very dubious assumption given the various values that elevel
> can have.  Maybe it's accidentally true -- I don't recall any
> reports of trouble here -- but it sure looks fragile.
>
> Hence, proposed patch attached.

Looks good to me.

--
Tristan Partin
Neon (https://neon.tech)