Implement message interrupt mode

[filmor]

Fixes #503.

This switches entirely over to message mode. We can either make this configurable (though it must be set for Windows) or remove the signal mode completely.

/EDIT: After adjusting to the PR comments, message mode is only used on Windows.

[stevengj]

I'm confused about how this is supposed to work. If an execute_request is being handled, then in our current implementation the eventloop will be hung until that completes, so it will never get the interrupt_request, no?

[filmor]

The message comes in via the control socket which is separate from the normal shell socket that gets the messages for execute, complete and other "normal" requests. Kernels are supposed to prioritise messages on the control socket.

[stevengj]

Oh, I missed that — yes, we handle the control socket in a separate asynchronous task, so that should be fine.

In the current Julia release, tasks are scheduled cooperatively, so the execution task may be hogging all of the cycles if it refuses to yield (or do something like I/O that implicitly yields), in which case this still won't work. But in a future Julia release the tasks will be able to run in separate threads (JuliaLang/julia#22631).

[filmor]

I guess this is still the best we can do for Windows :). It works reliably (on both Windows and Linux) for a simple sleep, I haven't tried it yet with actual load.

[stevengj]

(sleep lets other tasks run.)

[filmor]

Is there anything missing?

[stevengj]

Thanks!

[carstenbauer]

Great to see this fixed! @stevengj, is this worth tagging a release?

[stevengj]

Tagged 1.17.0