Support writing to block devices on MacOS

[aaronjeline]

If we get POLLNVAL, wake all of our readers/writers. MacOS will report POLLNVAL for any block device. Solves #757

[talex5]

I assume this means that, if the resource isn't ready, then it will spin? That's not a problem for /dev/null but it might be for other things.

Possibly that's still better than the current behaviour, but needs a comment in the code that it should be improved if it turns out to be a problem.

[aaronjeline]

but needs a comment in the code that it should be improved if it turns out to be a problem.

Added a note about this.

[talex5]

This results in us blocking on block devices

Does it block? I'd expect it to keep trying at 100% CPU instead (since we normally open things as non-blocking, it should get EAGAIN and try to wait again).

I wonder if we should just use select on macOS, since apparently you can avoid the 1024 FD limit there (with _DARWIN_UNLIMITED_SELECT), and I doubt people are using macOS to run e.g. high-performance servers with huge numbers of connections anyway.

Any other mac users have thoughts about this?

[aaronjeline]

I wonder if we should just use select on macOS

Do you mean just switch from poll() to select() for all files, or keep a separate thread up for monitoring non-pollable files with select.

I believe (2) is what libuv does

[talex5]

Either would be fine I think.

[aaronjeline]

Latest commits spawns a new thread to handle block devices on macOS. Let me know if you want anything organized differently. Happy to adjust as you'd like.