Horton

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

Jelte Fennema-Nio <jelte.fennema@microsoft.com>

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From: Jelte Fennema <Jelte.Fennema@microsoft.com>
To: Andres Freund <andres@anarazel.de>
Cc: Justin Pryzby <pryzby@telsasoft.com>, Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2022-06-27T09:29:39Z
Lists: pgsql-hackers

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From: Andres Freund <andres@anarazel.de>
> On 2022-04-04 15:21:54 +0000, Jelte Fennema wrote:
> > 2. Added some extra sleeps to the cancellation test, to remove random
> > failures on FreeBSD.
> 
> That's extremely extremely rarely the solution to address test reliability
> issues. It'll fail when running test under valgrind etc.
> 
> Why do you need sleeps / can you find another way to make the test reliable?

The problem they are solving is racy behaviour between sending the query
and sending the cancellation. If the cancellation is handled before the query
is started, then the query doesn't get cancelled. To solve this problem I used
the sleeps to wait a bit before sending the cancelation request.

When I wrote this, I couldn't think of a better way to do it then with sleeps.
But I didn't like it either (and I still don't). These emails made me start to think
again, about other ways of solving the problem. I think I've found another 
solution (see attached patch). The way I solve it now is by using another 
connection to check the state of the first one.

Jelte

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.