Horton

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

Tom Lane <tgl@sss.pgh.pa.us>

View thread View on postgr.es
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Jelte Fennema-Nio <postgres@jeltef.nl>, Noah Misch <noah@leadboat.com>, Jacob Champion <jacob.champion@enterprisedb.com>, Denis Laxalde <denis.laxalde@dalibo.com>, vignesh C <vignesh21@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, Greg Stark <stark@mit.edu>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Jelte Fennema <Jelte.Fennema@microsoft.com>, Daniel Gustafsson <daniel@yesql.se>, Peter Eisentraut <peter@eisentraut.org>, Andres Freund <andres@anarazel.de>, Justin Pryzby <pryzby@telsasoft.com>, Robert Haas <robertmhaas@gmail.com>, pgsql-hackers@lists.postgresql.org, Etsuro Fujita <etsuro.fujita@gmail.com>
Date: 2024-08-30T19:21:25Z
Lists: pgsql-hackers

Attachments

Alexander Lakhin <exclusion@gmail.com> writes:
> Let me show you another related anomaly, which drongo kindly discovered
> recently: [1]. That test failed with:
>   SELECT dblink_cancel_query('dtest1');
> - dblink_cancel_query
> ----------------------
> - OK
> +   dblink_cancel_query
> +--------------------------
> + cancel request timed out
>   (1 row)

While we're piling on, has anyone noticed that *non* Windows buildfarm
animals are also failing this test pretty frequently?  The most recent
occurrence is at [1], and it looks like this:

diff -U3 /home/bf/bf-build/mylodon/HEAD/pgsql/contrib/postgres_fdw/expected/query_cancel.out /home/bf/bf-build/mylodon/HEAD/pgsql.build/testrun/postgres_fdw/regress/results/query_cancel.out
--- /home/bf/bf-build/mylodon/HEAD/pgsql/contrib/postgres_fdw/expected/query_cancel.out	2024-07-22 11:09:50.638133878 +0000
+++ /home/bf/bf-build/mylodon/HEAD/pgsql.build/testrun/postgres_fdw/regress/results/query_cancel.out	2024-08-30 06:28:01.971083945 +0000
@@ -17,4 +17,5 @@
 SET LOCAL statement_timeout = '10ms';
 select count(*) from ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5; -- this takes very long
 ERROR:  canceling statement due to statement timeout
+WARNING:  could not get result of cancel request due to timeout
 COMMIT;

I trawled the buildfarm database for other occurrences of "could not
get result of cancel request" since this test went in.  I found 34
of them (see attachment), and none that weren't the timeout flavor.

Most of the failing machines are not especially slow, so even though
the hard-wired 30 second timeout that's being used here feels a little
under-engineered, I'm not sure that arranging to raise it would help.
My spidey sense feels that there's some actual bug here, but it's hard
to say where.  mylodon's postmaster log confirms that the 30 seconds
did elapse, and that there wasn't anything much else going on:

2024-08-30 06:27:31.926 UTC client backend[3668381] pg_regress/query_cancel ERROR:  canceling statement due to statement timeout
2024-08-30 06:27:31.926 UTC client backend[3668381] pg_regress/query_cancel STATEMENT:  select count(*) from ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5;
2024-08-30 06:28:01.946 UTC client backend[3668381] pg_regress/query_cancel WARNING:  could not get result of cancel request due to timeout

Any thoughts?

			regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mylodon&dt=2024-08-30%2006%3A25%3A46

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.