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

On Fri, Dec 29, 2023 at 1:38 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Jelte Fennema-Nio <me@jeltef.nl> writes:
> > 1. Protocol messages are much easier to inspect for connection poolers
> > than queries
>
> Unless you somehow forbid clients from setting GUCs in the old way,
> exactly how will that help a pooler?

I agree that for this to work out we need the things that you can set
this way to be able to be set in only this way. But I'm also a huge
fan of the idea -- if done correctly, it would solve the problem of an
end client sneaking SET ROLE or SET SESSION AUTHORIZATION past the
pooler, which is a huge issue that we really ought to address.

> > 2. It paves the way for GUCs that can only be set using a protocol
> > message (and not using SET).
>
> This is assuming facts not in evidence.  Why would we want such a thing?

See above.

> > 3. Being able to change GUCs while in an aborted transaction.
>
> I'm really dubious that that's sane.  How will it interact with, for
> example, changes to the same GUC done in the now-failed transaction?
> Or for that matter, changes that happen later in the current
> transaction?  It seems like you can't even think about this unless
> you deem GUC changes made this way to be non-transactional, which
> seems very wart-y and full of consistency problems.

I agree with these complaints.

> > 4. Have an easy way to use the value contained in a ParameterStatus
> > message as the value for a GUC
>
> I agree that GUC_LIST_QUOTE is a mess, but "I'm too lazy to deal
> with that" seems like a rather poor argument for instead expending
> the amount of labor involved in a protocol change.

Not sure about this one. It seems unwarranted to introduce an
accusation of laziness when someone is finally making the effort to
address what is IMV a pretty serious deficiency in our current
implementation, but I have no educated opinion about what if anything
ought to be done about GUC_LIST_QUOTE or how that relates to the
present effort.

> On the whole, this feels like you are trying to force some things
> into the GUC model that should not be there.  I do perceive that
> there are things that could be protocol-level variables, but
> trying to say they are a kind of GUC seems like it will create
> more problems than it solves.

This is not a bad thought. If we made the things that were settable
with this mechanism distinct from the set of things that are settable
as GUCs, that might work out better. For example, it completely the
objection to (3). But I'm not 100% sure, either.

-- 
Robert Haas
EDB: http://www.enterprisedb.com