Cancel a pending open() call

[DaveCausey]

I'm using aiofiles to read from a named pipe, which doesn't actually complete until the pipe is opened elsewhere for writing, so the read loop spends most of its time in that state. As far as I can tell, there is no way to cancel the open once it is called and clean up or gracefully exit the application.

[Tinche]

The way aiofiles works is by wrapping blocking calls in a function that uses threads. This unfortunately has inherent limitations, for example threads can't be portably and safely cancelled, unlike asyncio tasks. So I'm not sure what to do here.