On Mon, Nov 22, 2021 at 10:42 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Thomas Munro <thomas.munro@gmail.com> writes:
> > Hmm, maybe it's still not enough. Now that I have coffee, I thought
> > about the well known failure of idle_in_transaction_timeout to report
> > errors on Windows[1].
>
> Yeah, I think that may well be a manifestation of the same problem:
> once the backend exits, Winsock issues RST which prevents the client
> from reading the queued data. We had been analyzing that under the
> assumption that Windows obeys the TCP RFCs ... but having now been
> disabused of that optimism, it seems to match up pretty well.
> It'd be useful to check if Lars' patch cures that symptom.
Yeah, it sounds like it might solve at least the server-side problem.
Let's call that weird behaviour #1: RST on process exit. (I wonder if
my keep-the-socket-open-in-another-process thought experiment is
theoretically better: a lingering socket should be capable of
resending data that hasn't been ack'd yet in FIN-WAIT-1 state after
close, which I suspect might not happen if the TCP stack nukes the
socket. If close() avoids the proactive RST but still doesn't really
follow the shutdown protocol then it's papering over a crack in the
wall, but I'm not planning to argue about that...)
IIUC we'd still have weird behaviour #2 on the client side: TCP stack
drops buffered received data on the floor on receipt of RST.
So yeah, it'd be interesting to know if by avoiding/hiding weird
behaviour #1, idle_in_transaction_timeout works as desired most of the
time by tilting the race in favour of eager clients and favourable
scheduling. If a client sends a new query and then immediately begins
to read the response, there's a good chance it'll be able to read the
already-buffered error message before the query->RST ping pong...
Which I now understand is exactly what Lars was explaining: that sync
APIs (like the psql command shown in that other thread) might have a
good chance of winning that race, but for async APIs, the author of
the async API has no idea what its client is going to do.