Re: Feature Proposal: Connection Pool Optimization - Change the Connection User

On Sat, 2021-11-20 at 16:16 -0500, Tom Lane wrote:
> One more point is that the proposed business about
>
> * ImpersonateDatabaseUser will either succeed silently (0-RTT), or
> fail. Upon failure, no further commands will be processed until
> ImpersonateDatabaseUser succeeds.
>
> seems to require adding a huge amount of complication on the server side,
> and complication in the protocol spec itself, to save a rather minimal
> amount of complication in the middleware. Why can't we just say that
> a failed "impersonate" command leaves the session in the same state
> as before, and it's up to the pooler to do something about it? We are
> in any case trusting the pooler not to send commands from user A to
> a session logged in as user B.

When combined with the 0-RTT goal, I think a silent ignore would just
invite more security problems. Todd is effectively proposing packet
pipelining, so the pipeline has to fail shut.

A more modern approach might be to attach the authentication to the
packet itself (e.g. cryptographically, with a MAC), if the goal is to
enable per-statement authentication anyway. In theory that turns the
middleware into a message passer instead of a confusable deputy. But it
requires more complicated setup between the client and server.

> PS: I wonder how we test such a feature meaningfully without
> incorporating a pooler right into the Postgres tree. I don't
> want to do that, for sure.

Having protocol-level tests for bytes on the wire would not only help
proposals like this but also get coverage for a huge number of edge
cases. Magnus has added src/test/protocol for the server, written in
Perl, in his PROXY proposal. And I've added a protocol suite for both
the client and server, written in Python/pytest, in my OAuth proof of
concept. I think something is badly needed in this area.

--Jacob