Horton

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

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Jelte Fennema-Nio <postgres@jeltef.nl>, Noah Misch <noah@leadboat.com>, Tom Lane <tgl@sss.pgh.pa.us>, 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: 2024-07-16T15:08:52Z
Lists: pgsql-hackers 
On 2024-Jul-16, Alexander Lakhin wrote:

> I've managed to reproduce this issue in my Cygwin environment by running
> the postgres_fdw test in a loop (10 iterations are enough to get the
> described effect). And what I'm seeing is that a query-cancelling backend
> is stuck inside pgfdw_xact_callback() -> pgfdw_abort_cleanup() ->
> pgfdw_cancel_query() -> pgfdw_cancel_query_begin() -> libpqsrv_cancel() ->
> WaitLatchOrSocket() -> WaitEventSetWait() -> WaitEventSetWaitBlock() ->
> poll().
> 
> The timeout value (approximately 30 seconds), which is passed to poll(),
> is effectively ignored by this call — the waiting lasts for unlimited time.

Ugh.  I tried to follow what's going on in that cygwin code, but I gave
up pretty quickly.  It depends on a mutex, but I didn't see the mutex
being defined or initialized anywhere.

> So it looks like a Cygwin bug, but maybe something should be done on our side
> too, at least to prevent such lorikeet failures.

I don't know what else we can do other than remove the test.

Maybe we can disable this test specifically on Cygwin.  We could do that
by creating a postgres_fdw_cancel.sql file, with the current output for
all platforms, and a "SELECT version() ~ 'cygwin' AS skip_test" query,
as we do for encoding tests and such.

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
"Doing what he did amounts to sticking his fingers under the hood of the
implementation; if he gets his fingers burnt, it's his problem."  (Tom Lane)

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.