[Bug 258412] kevent EVFILT_TIMER, EV_DISPATCH lost if event is re-enabled

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=258412

            Bug ID: 258412
           Summary: kevent EVFILT_TIMER, EV_DISPATCH lost if event is
                    re-enabled
           Product: Base System
           Version: 13.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: a.cudbardb@freeradius.org

Currently expanding libkqueue tests.  libkqueue implements kqueue for Linux,
Solaris and Windows as a userland library.

One test - test_kevent_timer_dispatch - appears to fail on both macOS 11.5.2
and FreeBSD 13 but for different reasons.

Test steps are below:

- Add a new timer, EV_SET(kq, 4, EVFILT_TIMER, EV_ADD, EV_DISPATCH, 0, 200,
NULL)
- Call kevent, wait for timer to fire.
- Verify one kevent was returned the the correct flags and data field set to 1.
- Wait 500ms.
- Verify no more kevents are available.

- Re-enable timer, EV_SET(kq, 4, EVFILT_TIMER, EV_ENABLE | EV_DISPATCH, 0, 200,
NULL)
- Verify no events are generated instantaneously.

The above step fails on FreeBSD 13, we get one event with the data field set to
1.

- Wait 1s
- Check that exactly 1 event has fired.

The above step fails on macOS, we get one event with the data field set to 5.

Adding EV_ADD to the call re-enabling the timer does nothing, as does only
passing EV_ENABLE.

To run the test suite containing the failing test:

  git clone https://github.com/mheily/libkqueue.git
  cd libkqueue
  cmake . test/CMakeLists.txt -DWITH_NATIVE_KQUEUE_BUGS=1
  make -C test/
  ./test/libkqueue-test timer

You'll need the cmake, git, and gcc or llvm pkgs installed.  There are no other
dependencies.

The actual test is available here:
https://github.com/mheily/libkqueue/blob/935ebc39f78d2e06ce0d20f6c297e4b2b7e4c1bc/test/timer.c#L122

I believe FreeBSD pulled in libkqueue's test suite at some point in the distant
past.  The test framework hasn't changed much in the past 10 years so you may
be able to just drop the updated test in.

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