Horton

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

Jelte Fennema-Nio <postgres@jeltef.nl>

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From: Jelte Fennema <postgres@jeltef.nl>
To: Denis Laxalde <denis.laxalde@dalibo.com>
Cc: Greg Stark <stark@mit.edu>, Tom Lane <tgl@sss.pgh.pa.us>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Jelte Fennema <Jelte.Fennema@microsoft.com>, Daniel Gustafsson <daniel@yesql.se>, Peter Eisentraut <peter@eisentraut.org>, Jacob Champion <jchampion@timescale.com>, Andres Freund <andres@anarazel.de>, Justin Pryzby <pryzby@telsasoft.com>, Robert Haas <robertmhaas@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2023-03-28T15:54:06Z
Lists: pgsql-hackers

Attachments

On Tue, 28 Mar 2023 at 16:54, Denis Laxalde <denis.laxalde@dalibo.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?

Commits

  1. postgres_fdw: re-issue cancel requests a few times if necessary.

  2. Make postgres_fdw's query_cancel test less flaky.

  3. postgres_fdw: Split out the query_cancel test to its own file

  4. Fix copy-paste mistake in PQcancelCreate

  5. Make libpqsrv_cancel's return const char *, not char *

  6. Stabilize postgres_fdw test

  7. libpq-be-fe-helpers.h: wrap new cancel APIs

  8. dblink/isolationtester/fe_utils: Use new cancel API

  9. Put libpq_pipeline cancel test back

  10. Hopefully make libpq_pipeline's new cancel test more reliable

  11. libpq: Add encrypted and non-blocking query cancellation routines

  12. libpq: Move pg_cancel to fe-cancel.c

  13. Add tests for libpq query cancellation APIs

  14. Add missing connection statuses to docs

  15. libpq: Change some static functions to extern

  16. libpq: Add pqReleaseConnHosts function

  17. libpq: Move cancellation related functions to fe-cancel.c

  18. Make spelling of cancelled/cancellation consistent

  19. Be more wary about OpenSSL not setting errno on error.

  20. libpq: Use modern socket flags, if available.

  21. Drop test view when done with it.

  22. Doc: add some doco about using the libpq_pipeline test module.

  23. postgres_fdw: Allow cancellation of transaction control commands.