[Bug 258310] kevent() does not see signal with zero timeout

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=258310

            Bug ID: 258310
           Summary: kevent() does not see signal with zero timeout
           Product: Base System
           Version: 13.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: arichardson@FreeBSD.org

While adding patches to natively support FreeBSD in wayland, I noticed that
epoll-shim (used to emulate epoll with kqueue) didn't report signals sent to
self via kill() unless I added a non-zero timeout (even in a single-threaded
test program). In https://github.com/jiixyj/epoll-shim/pull/32 
Jan Kokemüller provided a reduced test case using only kqueue and kevent:

```
#ifdef NDEBUG
#undef NDEBUG
#endif

#define _GNU_SOURCE

#include <sys/types.h>

#include <sys/event.h>

#include <assert.h>
#include <errno.h>
#include <signal.h>
#include <stdlib.h>

#include <poll.h>
#include <unistd.h>

int
main(void)
{
        int rv;

        sigset_t set;
        rv = sigemptyset(&set);
        assert(rv == 0);
        rv = sigaddset(&set, SIGUSR1);
        assert(rv == 0);

        rv = sigprocmask(SIG_BLOCK, &set, NULL);
        assert(rv == 0);

        int skq = kqueue();
        assert(skq >= 0);

        struct kevent kev;
        EV_SET(&kev, SIGUSR1, EVFILT_SIGNAL, EV_ADD, 0, 0, 0);
        rv = kevent(skq, &kev, 1, NULL, 0, NULL);
        assert(rv == 0);

        int kq = kqueue();
        assert(kq >= 0);

        EV_SET(&kev, skq, EVFILT_READ, EV_ADD | EV_CLEAR, 0, 0, 0);
        rv = kevent(kq, &kev, 1, NULL, 0, NULL);
        assert(rv == 0);

        for (;;) {
                rv = kill(getpid(), SIGUSR1);
                assert(rv == 0);

                /* Turn this into `#if 1` to avoid the race. */
#if 0
                rv = kevent(kq, NULL, 0, &kev, 1, NULL);
#else
                rv = kevent(kq, NULL, 0, &kev, 1, &(struct timespec) { 0, 0 });
#endif
                assert(rv == 1);
                rv = kevent(kq, NULL, 0, &kev, 1, &(struct timespec) { 0, 0 });
                assert(rv == 0);

                rv = kevent(skq, NULL, 0, &kev, 1, &(struct timespec) { 0, 0
});
                assert(rv == 1);
                rv = kevent(skq, NULL, 0, &kev, 1, &(struct timespec) { 0, 0
});
                assert(rv == 0);

                siginfo_t siginfo;

                rv = sigtimedwait(&set, &siginfo, &(struct timespec) { 0, 0 });
                assert(rv == SIGUSR1);

                rv = sigtimedwait(&set, &siginfo, &(struct timespec) { 0, 0 });
                assert(rv < 0);
                assert(errno == EAGAIN);
        }
}
```

Is this behaviour expected or a bug?

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