Re: Protocol forced to V2 in low-memory conditions?

[ still catching up on old email ]

I wrote:
> Andrew Dunstan <andrew@dunslane.net> writes:
>> On 09/11/2013 02:30 PM, Robert Haas wrote:
>>> On Tue, Sep 10, 2013 at 9:53 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>>> Note that I was proposing removing libpq's support for V2 connections.
>>>> Not the backend's.

>>> I vote against this.  If we remove V2 support from libpq, then we'll
>>> have no easy way to test that the backend's support still works.

>> How is it tested now, and who is doing the testing?

> Exactly.  The current support in libpq is nigh useless for testing
> purposes, because there's no way to activate that code path on command.
> It only runs if libpq (thinks it) is connecting to a pre-7.4 backend.

Actually ... there's another way we might deal with this.  Suppose we
invent a libpq connection option to specify whether to use v2 or v3
protocol, defaulting to the latter, and then just remove the protocol-
fallback-during-connection code path.  If there is anyone out there using
a modern libpq to talk to a pre-7.4 server, they can be told to invoke
the connection option.  This gets rid of the unexpected-protocol-downgrade
problem in a reliable manner, and it also gives us a way to test V2 code
paths in both libpq and the backend, which Andrew is correct to finger as
something that goes nearly totally untested right now.

The main objections I can see to this are (1) it wouldn't provide
a back-patchable fix, and (2) it'd be adding more legacy code instead
of removing it.  But the other approaches that we've talked about didn't
sound very back-patchable either, so I think only (2) really holds
much water.
        regards, tom lane