Use Windows message-oriented pipes to create Channels

[richardsheridan]

Relevant to #1773, #1767, #824. Maybe even #959 and #1208?

This PR leverages Windows message-oriented pipes to create objects of type Channel[Bytes] (according to the current state of #1208), supplementing the existing Stream/Channel[Byte] classes. This came about as I was trying to find a way to asyncify multiprocessing.connection.Pipe for #1781, however the implementation stands totally separate from that. The internals of Pipe are very similar to asyncio.windows_utils.pipe except that the underlying objects are message-oriented rather than byte-oriented, meaning senders dump a whole message to the OS, and if receivers haven't allocated a big enough buffer, they get an "error", which is really just an invitation to get the rest with another, bigger buffer.

Working towards providing an API for #824, in addition to the class constructors it might make sense to provide make_pipe_stream and make_pipe_channel from test_windows_pipes, since it is quite easy to get a windows pipe handle that is in the wrong state. Or possibly add some checks into the class constructors to ensure the pipe is in the right configuration with SetNamedPipeHandleState? Also, users will need clear instructions as to how to get one end of the pipe into a subprocess. (I personally have no idea, I've been letting multiprocessing do its magic.)

TODO:

• [x] Write tests
• [ ] Expose public API?
• [ ] Write docs
• [ ] Write newsfragment

[codecov[bot]]

Codecov Report

Merging #1782 (0a4ba95) into master (58e9b80) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1782      +/-   ##
==========================================
+ Coverage   99.59%   99.64%   +0.05%     
==========================================
  Files         114      114              
  Lines       14574    14739     +165     
  Branches     1110     1118       +8     
==========================================
+ Hits        14515    14687     +172     
+ Misses         42       36       -6     
+ Partials       17       16       -1     

Impacted Files	Coverage Δ
trio/_core/_windows_cffi.py	100.00% <100.00%> (ø)
trio/_windows_pipes.py	100.00% <100.00%> (ø)
trio/tests/test_windows_pipes.py	100.00% <100.00%> (ø)
trio/testing/_check_streams.py	99.33% <0.00%> (+0.66%)	:arrow_up:
trio/tests/test_socket.py	100.00% <0.00%> (+0.81%)	:arrow_up:

[richardsheridan]

I think this PR is ready for review! not sure if CI needs to be refreshed somehow...

[richardsheridan]

Woke up in a cold sweat having dreamed of an obscure cancellation bug. I think this is a safe use of the shielded cancel scope, otherwise I'll have to expose a sync function to call ReadFile in _windows_cffi