Horton

Re: Add non-blocking version of PQcancel

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jacob Champion <pchampion@vmware.com>, "Jelte.Fennema@microsoft.com" <Jelte.Fennema@microsoft.com>, pgsql-hackers@lists.postgresql.org
Date: 2022-03-24T22:49:09Z
Lists: pgsql-hackers 
Hi,

On 2022-03-24 17:41:53 -0400, Tom Lane wrote:
> I kind of feel that this patch is going in the wrong direction.
> I do see the need for a version of PQcancel that can encrypt the
> transmitted cancel request (and yes, that should work on the backend
> side; see recursion in ProcessStartupPacket).  I have not seen
> requests for a non-blocking version, and this doesn't surprise me.
> I feel that the whole non-blocking aspect of libpq probably belongs
> to another era when people didn't trust threads.

That's not a whole lot of fun if you think of cases like postgres_fdw (or
citus as in Jelte's case), which run inside the backend. Even with just a
single postgres_fdw, we don't really want to end up in an uninterruptible
PQcancel() that doesn't even react to pg_terminate_backend().

Even if using threads weren't an issue, I don't really buy the premise - most
networking code has moved *away* from using dedicated threads for each
connection. It just doesn't scale.


Leaving PQcancel aside, we use the non-blocking libpq stuff widely
ourselves. I think walreceiver, isolationtester, pgbench etc would be *much*
harder to get working equally well if there was just blocking calls. If
anything, we're getting to the point where purely blocking functionality
shouldn't be added anymore.

Greetings,

Andres Freund

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.