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

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