macOS 10.5 needs a fallback for `libdispatch` and `NOTE_TRIGGER`

[barracuda156]

While #57 fixes the build for 10.6, it is not sufficient for 10.5, due to:

1. libdispatch does not exist in 10.5 and, unfortunately, does not build from source either, at least so far. Can mach semaphores be used as a fallback?
2. 10.5 has no NOTE_TRIGGER or EV_TRIGGER, and while the existing code provides a fallback for this case, it fails on macOS, since there is no pipe2. There was a similar issue elsewhere, which was solved by slightly changing the implementation: epoupon/lms@777c360

[jiixyj]

At this point in time, OS X 10.5 is more than 10 years out of support. I would prefer to not add more hacks for operating systems that old. Even supporting/testing on current macOS is basically impossible for me outside of using the GitHub Action runners since I'm not macOS user.

In hindsight, even adding support for "modern" macOS was probably a mistake. Look at all the compat_ files in https://github.com/jiixyj/epoll-shim/tree/master/src needed to emulate various POSIX functionality on macOS. For example, the pipe2 API appeared more than 16 years ago, and was adopted by basically all Unix like systems and even by the newest edition of POSIX. Except OS X/macOS.

libdispatch does not exist in 10.5 and, unfortunately, does not build from source either, at least so far. Can mach semaphores be used as a fallback?

I believe I used sem_ only for the rwlock implementation. Maybe you can find a different rwlock implementation? The only important thing is that rwlock_downgrade needs to be supported. This operation transforms a held writer lock into a readers lock. Another option might be to avoid using rwlock entirely and just use a normal pthread mutex instead. This would make the performance a bit worse, but it might still be enough for your use case.

10.5 has no NOTE_TRIGGER or EV_TRIGGER, and while the existing code provides a fallback for this case, it fails on macOS, since there is no pipe2.

You might be able to use the pipe2 implementation in compat_pipe2.c for this. You should be able to enable the pipe2 polyfill for a target by linking $<BUILD_INTERFACE:compat_enable_pipe2> to it in the CMakeLists.txt.