pgsql: oauth: Remove stale events from the kqueue multiplexer

oauth: Remove stale events from the kqueue multiplexer

If a socket is added to the kqueue, becomes readable/writable, and
subsequently becomes non-readable/writable again, the kqueue itself will
remain readable until either the socket registration is removed, or the
stale event is cleared via a call to kevent().

In many simple cases, Curl itself will remove the socket registration
quickly, but in real-world usage, this is not guaranteed to happen. The
kqueue can then remain stuck in a permanently readable state until the
request ends, which results in pointless wakeups for the client and
wasted CPU time.

Implement comb_multiplexer() to call kevent() and unstick any stale
events that would cause unnecessary callbacks. This is called right
after drive_request(), before we return control to the client to wait.

Suggested-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Co-authored-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Backpatch-through: 18
Discussion: https://postgr.es/m/CAOYmi+nDZxJHaWj9_jRSyf8uMToCADAmOfJEggsKW-kY7aUwHA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ff5b0824b3b0c58bbb322719628c42b7a4751dd9

Modified Files
--------------
src/interfaces/libpq-oauth/oauth-curl.c | 67 ++++++++++++++++++++++++++++++---
1 file changed, 61 insertions(+), 6 deletions(-)