Horton

Re: [PATCH] Fix ALTER SYSTEM empty string bug for GUC_LIST_QUOTE parameters

Andrei Klychkov <andrew.a.klychkov@gmail.com>

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From: Andrei Klychkov <andrew.a.klychkov@gmail.com>
To: Jim Jones <jim.jones@uni-muenster.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Fujii Masao <masao.fujii@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-09-05T09:08:15Z
Lists: pgsql-hackers

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Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Allow "SET list_guc TO NULL" to specify setting the GUC to empty.

Attachments

As it solves the initial issue, it SGTM too.
I applied v3, updated the docs and added some tests in attached v4.
Hopefully it's OK.
Please take a look

Thanks
Regards

On Fri, Sep 5, 2025 at 9:33 AM Jim Jones <jim.jones@uni-muenster.de> wrote:

>
>
> On 04.09.25 23:52, Tom Lane wrote:
> > Note that the patch includes changing SplitIdentifierString and its
> > clones to forbid zero-length quoted elements, which were formerly
> > allowed.  Without this, we'd accept values from config files that
> > could not be represented in SET, which is exactly the situation we
> > are trying to fix.
> >
> > I'm not entirely sure if this is the way to go, or if we want to
> > adopt some other solution that doesn't involve forbidding empty
> > list elements.  I suspect that anything else we come up with would
> > be less intuitive than letting SET list_var = '' do the job;
> > but maybe I just lack imagination today.
>
> This approach LGTM. It solves the initial issue:
>
> $ psql postgres -c "ALTER SYSTEM SET local_preload_libraries TO '';"
> ALTER SYSTEM
>
> $ cat /usr/local/postgres-dev/testdb/postgresql.auto.conf
> # Do not edit this file manually!
> # It will be overwritten by the ALTER SYSTEM command.
> local_preload_libraries = ''
>
> ... making a clear distinction between empty elements and empty lists:
>
> postgres=# SET local_preload_libraries TO '','foo';
> ERROR:  SET local_preload_libraries does not accept empty-string elements
>
> postgres=# SET local_preload_libraries TO '';
> SET
> postgres=# SHOW local_preload_libraries;
>  local_preload_libraries
> -------------------------
>
> (1 row)
>
> The ambiguity between an empty list and an empty element has always
> existed in list-valued GUCs. This patch resolves the issue by
> disallowing empty elements, thereby making '' an unambiguous
> representation of an empty list. Personally, I find SET var TO NULL (or
> perhaps a keyword like EMPTY or NONE) a more palatable syntax for
> expressing empty lists in this case. However, I’m not sure the
> additional complexity and compatibility implications would justify such
> a change.
>
> Best regards, Jim
>