Horton

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: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2022-01-12T15:22:18Z
Lists: pgsql-hackers

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The existing PQcancel API is using blocking IO. This makes PQcancel
impossible to use in an event loop based codebase, without blocking the
event loop until the call returns.

This patch adds a new cancellation API to libpq which is called
PQcancelConnectionStart. This API can be used to send cancellations in a
non-blocking fashion. To do this it internally uses regular PGconn
connection establishment. This has as a downside that
PQcancelConnectionStart cannot be safely called from a  signal handler.

Luckily, this should be fine for most usages of this API. Since most
code that's using an event loop handles signals in that event loop as
well (as opposed to calling functions from the signal handler directly).

There are also a few advantages of this approach:
1. No need to add and maintain a second non-blocking connection
   establishment codepath.
2. Cancel connections benefit automatically from any improvements made
   to the normal connection establishment codepath. Examples of things
   that it currently gets for free currently are TLS support and
   keepalive settings.

This patch also includes a test for this new API (and also the already
existing cancellation APIs). The test can be easily run like this:

    cd src/test/modules/libpq_pipeline
    make && ./libpq_pipeline cancel

NOTE: I have not tested this with GSS for the moment. My expectation is
that using this new API with a GSS connection will result in a
CONNECTION_BAD status when calling PQcancelStatus. The reason for this
is that GSS reads will also need to communicate back that an EOF was
found, just like I've done for TLS reads and unencrypted reads. Since in
case of a cancel connection an EOF is actually expected, and should not
be treated as an error.

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.