Re: Add new protocol message to change GUCs for usage with future protocol-only GUCs

On Fri, 16 Aug 2024 at 00:39, Jacob Champion
<jacob(dot)champion(at)enterprisedb(dot)com> wrote:
>
> On Thu, Aug 15, 2024 at 3:04 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> > Perhaps we should even change it to return
> > 300000 for protocol version 3.0, and just leave a note in the docs like
> > "in older versions of libpq, this returned 3 for protocol version 3.0".
>
> I think that would absolutely break current code. It's not uncommon
> (IME) for hand-built clients wrapping libpq to make sure they're not
> talking v2 before turning on some feature, and they're allowed to do
> that with a PQprotocolVersion() == 3 check. A GitHub code search
> brings up examples.

Can you give a link for that code search and/or an example where
someone used it like that in a real setting? The only example I could
find where someone used it at all was psycopg having a unittest for
their python wrapper around this API, and they indeed used == 3.

> As for 30001: I don't see the value in modifying an exported API in
> this way. Especially since we can add a new entry point that will be
> guaranteed not to break anyone, as Robert suggested. I think it's a
> POLA violation at minimum; my understanding was that up until this
> point, the value was incremented during major (incompatible) version
> bumps. And I think other users will have had the same understanding.

The advantage is not introducing yet another API when we already have
one with a great name that no-one is currently using. The current API
is in practice just a very convoluted way of writing 3. Also doing an
== 3 check is obviously problematic, if people use this function they
should be using > 3 to be compatible with future versions. So if we
ever introduce protocol version 4, then these (afaict theoretical)
users would break anyway.