Re: [EXTERNAL] Re: Add non-blocking version of PQcancel

On Tue, 28 Mar 2023 at 16:54, Denis Laxalde <denis(dot)laxalde(at)dalibo(dot)com> wrote:
> I had a look into your patchset (v16), did a quick review and played a
> bit with the feature.
>
> Patch 2 is missing the documentation about PQcancelSocket() and contains
> a few typos; please find attached a (fixup) patch to correct these.

Thanks applied that patch and attached a new patchset

> Namely, I wonder why it returns a PGcancelConn and what's the
> point of requiring the user to call PQcancelStatus() to see if something
> got wrong. Maybe it could be defined as:
>
> int PQcancelSend(PGcancelConn *cancelConn);
>
> where the return value would be status? And the user would only need to
> call PQcancelErrorMessage() in case of error. This would leave only one
> single way to create a PGcancelConn value (i.e. PQcancelConn()), which
> seems less confusing to me.

To clarify what you mean, the API would then be like this:
PGcancelConn cancelConn = PQcancelConn(conn);
if (PQcancelSend(cancelConn) == CONNECTION_BAD) {
printf("ERROR %s\n", PQcancelErrorMessage(cancelConn))
exit(1)
}

Instead of:
PGcancelConn cancelConn = PQcancelSend(conn);
if (PQcancelStatus(cancelConn) == CONNECTION_BAD) {
printf("ERROR %s\n", PQcancelErrorMessage(cancelConn))
exit(1)
}

Those are so similar, that I have no preference either way. If more
people prefer one over the other I'm happy to change it, but for now
I'll keep it as is.

> As part of my testing, I've implemented non-blocking cancellation in
> Psycopg, based on v16 on this patchset. Overall this worked fine and
> seems useful; if you want to try it:
>
> https://github.com/dlax/psycopg3/tree/pg16/non-blocking-pqcancel

That's great to hear! I'll try to take a closer look at that change tomorrow.

> (The only thing I found slightly inconvenient is the need to convey the
> connection encoding (from PGconn) when handling error message from the
> PGcancelConn.)

Could you expand a bit more on this? And if you have any idea on how
to improve the API with regards to this?