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Date: Wed, 27 Dec 2023 12:26:42 -0600
Message-Id: <CXZBTBI4CC14.19ODDAQ7T9KMB@neon.tech>
From: "Tristan Partin" <tristan@neon.tech>
Subject: Re: Two small bugs in guc.c
To: "Tom Lane" <tgl@sss.pgh.pa.us>, <pgsql-hackers@lists.postgresql.org>
References: <2089235.1703617353@sss.pgh.pa.us>
In-Reply-To: <2089235.1703617353@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/CXZBTBI4CC14.19ODDAQ7T9KMB%40neon.tech>
Precedence: bulk

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 change=
d in
>                  * the config file, we want to accept the new value in th=
e
>                  * postmaster (whence it will propagate to
>                  * subsequently-started backends), but ignore it in exist=
ing
>                  * 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 =3D false;
>
> While studying this, I also noted that the bit to prevent changes in
> parallel workers seems seriously broken:
>
>     if (IsInParallelMode() && changeVal && action !=3D GUC_ACTION_SAVE)
>         ereport(elevel,
>                 (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
>                  errmsg("cannot set parameters during a parallel operatio=
n")));
>
> 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)