Horton

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: Justin Pryzby <pryzby@telsasoft.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Jacob Champion <pchampion@vmware.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-01T16:13:07Z
Lists: pgsql-hackers

Attachments

Attached is the latest version of this patch, which I think is now in a state
in which it could be merged. The changes are:

1. Don't do host and address discovery for cancel connections. It now 
   reuses raddr and whichhost from the original connection. This makes
   sure the cancel always goes to the right server, even when DNS records 
   change or another server would be chosen now in case of connnection
   strings containing multiple hosts.
2. Fix the windows CI failure. This is done by both using the threadsafe code 
   in the the dblink cancellation code, and also by not erroring a cancellation
   connection on windows in case of any errors. This last one is to work around
   the issue described in this thread:
   https://www.postgresql.org/message-id/flat/90b34057-4176-7bb0-0dbb-9822a5f6425b%40greiz-reinsdorf.de

I also went over most of the functions that take a PGconn, to see if they needed
extra checks to guard against being executed on cancel. So far all seemed fine,
either they should be okay to execute against a cancellation connection, or 
they failed already anyway because a cancellation connection never reaches
the CONNECTION_OK state. So I didn't add any checks specifically for cancel
connections. I'll do this again next week with a fresh head, to see if I haven't 
missed any cases.

I'll try to find some time early next week to implement non-blocking cancellation
usage in postgres_fdw, i.e. the bonus task I mentioned in my previous email. But 
I don't think it's necessary to have that implemented before merging.

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.