Re: libpqrcv_PQexec() seems to violate latch protocol

On 2017-06-06 17:14:59 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > The function in $subject does:
>
> > ResetLatch(&MyProc->procLatch);
> > rc = WaitLatchOrSocket(&MyProc->procLatch,
> > WL_POSTMASTER_DEATH | WL_SOCKET_READABLE |
> > WL_LATCH_SET,
> > PQsocket(streamConn),
> > 0,
> > WAIT_EVENT_LIBPQWALRECEIVER);
>
> Yeah, this is certainly broken.
>
> > Afaict, the ResetLatch() really should just instead be in the if (rc & WL_LATCH_SET) block.
>
> And, to be specific, it should be before the CHECK_FOR_INTERRUPTS call,
> since that is the useful work that we want to be sure occurs after
> any latch-setting event.

Right. I found a couple more instance of similarly iffy, although not
quite as broken, patterns in launcher.c. It's easy to get this wrong,
but it's a lot easy if you do it differently everywhere you use a
latch. It's not good if code in the same file, by the same author(s),
has different ways of using latches.

- Andres