Add support for mypy daemon on Windows

[ilevkivskyi]

There is a major limitation for mypy daemon and fine-grained incremental mode: mypy daemon doesn't work on Windows, see http://mypy.readthedocs.io/en/latest/mypy_daemon.html#limitations

In particular, dmypy_server.daemonize uses some functionalities not available on Windows (such as os.fork). It would be great to remove this limitation (and update Appveyor to test the support).

@ethanhs It looks like you have most experience on Windows among all of us, would you like to take this task?

[emmatyping]

@ilevkivskyi I have a few more things to follow up on for PEP 561, but after that I can take this.

[emmatyping]

Mostly for myself, but also those who are interested:

After looking at the implementation a bit and talking with Steve Dower (who was very helpful!) I think the best plan is to use named pipes (which act esesentially like Unix sockets on Windows) for communication and a detached process (using creation flags of Popen). This should be enough to approximate the current UNIX API used by the mypy daemon. We will end up using _winapi.

[emmatyping]

As an update, I have finally figured out the Windows API incantations needed to get a server/client setup working over named pipes, and can spawn a detached process which together essentially emulate much of the fork/Unix pipe aspects of the classic Unix daemon. I will start working on the actually mypy daemon now I know how I will implement it :)

[ilevkivskyi]

@ethanhs Sorry for disturbing you, just wanted to check if there are any updates on this. This problem just appeared again in other context.

Thanks for helping with this!

[emmatyping]

@ilevkivskyi no problem! I've been a bit side-tracked with some PEP 561 stuff, so nothing new to report sadly. I will get back to this soon hopefully.

[emmatyping]

Unfortunately, I do not think I will have time to work on this any time soon (at least until December most likely). I am going to un-assign myself so someone else can pick this up if they are interested, but if not, I will come back to this in December.

[JukkaL]

@ethanhs Do you happen to have any prototype/work-in-progress code you can share? It's okay if it's not very polished.