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

On Fri, 5 Jan 2024 at 18:08, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Jan 5, 2024 at 11:53 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > There is a lot of infrastructure we'll have to re-invent if
> > we make this completely independent of GUCs, notably:
> > * a way to establish the initial/default value
> > * a way to display the active value
> >
> > So my thought was that this should be implemented as an (unchangeable)
> > flag bit for a GUC variable, GUC_PROTOCOL_ONLY or something like that,
> > and then we would refuse SQL-based set attempts on that. The behavior
> > would end up being very much like PGC_BACKEND variables, in that we
> > could allow all the existing setting methods to work to establish
> > a session's initial value; but after that, it can only change within
> > that session via a protocol message from the client. With that
> > rule, it's okay for the protocol message to be nontransactional since
> > there's no interaction with transactions.
>
> Maybe, but it seems like it might be complicated to make that work
> with the existing GUC code. GUCs are fundamentally transactional, I
> think.

They are not fundamentally transactional afaict based on the changes
that were needed so far. It makes sense too, because e.g. SIGHUP
should change the GUC value if the config changed no matter if the
current transaction aborts or succeeds.

Based on my experience when writing the current set of patches I think
the GUC infrastructure fits quite well with protocol extension
parameters. When you add a new flag bit it feels very natural
(whatever you may call this flag GUC_PROTOCOL_ONLY,
GUC_PROTOCOL_EXTENSION, or something else).