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

On Fri, 5 Apr 2024 at 18:43, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I don't buy that argument, actually.  libpq, and pretty much every
> other client AFAIK, has provisions to let higher code levels insert
> random options into the startup packet.  So to make this work libpq
> would have to filter or at least inspect such options, which is
> logic that doesn't exist and doesn't seem nice to need.

libpq actually doesn't support doing this (only by putting them in the
"options" parameter, but _pq_ parameters would not be allowed there),
but indeed many other clients do allow this and indeed likely don't
have logic to filter/disallow _pq_ prefixed parameters.

This seems very easy to address though: Only parse _pq_ options when
protocol version 3.1 is requested by the client, and otherwise always
report them as "not supported". Then clients upgrading to 3.1, they
should filter/disallow _pq_ parameters to be arbitrarily set. I don't
think that's hard/not nice to add, it's literally a prefix check for
the "_pq_." string.

> The other problem with adding these things in the startup packet
> is that when you send that packet, you don't know what the server
> version is and hence don't know if it will take these options.

(imho) the whole point of the _pq_ options is that they don't trigger
an error when they are requested by the client, but not supported by
the server. So I don't understand your problem here.

> What's so bad about insisting that these options must be sent in a
> separate message?

To not require an additional roundtrip waiting for the server to respond.