kern/58916: timerfd(2) claims ready for write

[campbell+netbsd%mumble.net@localhost]

>Number:         58916
>Category:       kern
>Synopsis:       timerfd(2) claims ready for write
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Dec 18 14:55:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10
>Organization:
The TimerFD Writation
>Environment:
>Description:
For a timerfd(2), select(2) returns writable if asked and poll(2) returns POLLOUT|POLLWRNORM if asked.  (Also POLLRDNORM if asked.)

In contrast, in Linux, select(2) never returns writable and poll(2) never returns POLLOUT|POLLWRNORM -- or, for that matter, anything other than POLLIN.

Writing to a timerfd fails with EOPNOTSUPP, so it's not useful to claim writable.  This doesn't appear to be a POSIX interface, so it looks like Linux is the `spec' here if I haven't missed anything obvious.
>How-To-Repeat:
run Python 3.13.1 test suite

 ======================================================================
 FAIL: test_timerfd_ns_select (test.test_os.TimerfdTests.test_timerfd_ns_select)
 ----------------------------------------------------------------------
 Traceback (most recent call last):
   File "/scratch/lang/python313/work/Python-3.13.1/Lib/test/test_os.py", line 4436, in test_timerfd_ns_select
     self.assertEqual((rfd, wfd, xfd), ([], [], []))
     ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 AssertionError: Tuples differ: ([], [3], []) != ([], [], [])

 First differing element 1:
 [3]
 []

 - ([], [3], [])
 ?       -

 + ([], [], [])
>Fix:
Yes, please!