Add trio.open_process, and deprecate Process(...)

[njsmith]

Fixes gh-1109

[njsmith]

Oh bleh, Windows wants to do async stuff in Process.__init__. So instead of shoving the whole thing off into a thread we have to shove just the Popen call into a thread and move the rest into open_process.

[njsmith]

...which is actually pretty non-trivial, if we want to share the rest of the setup code between open_process and the legacy __init__ code during the deprecation period. I'm going to go to bed and see if I have any better ideas in the morning...

[oremanj]

What's the additional async stuff that Windows needs in Process.init? Is it the creation of the named pipes?

[njsmith]

yeah, and specifically registering them with IOCP, which requires finding the io manager's state

[oremanj]

oh I see, not so much "wants to do blocking stuff" as "wants to do stuff in the Trio thread". Yeah that's a tough one :-( Will mull it over some.

[codecov]

Codecov Report

Merging #1113 into master will decrease coverage by 5.45%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1113      +/-   ##
==========================================
- Coverage   99.51%   94.05%   -5.46%     
==========================================
  Files         102      102              
  Lines       12519    12536      +17     
  Branches      953      953              
==========================================
- Hits        12458    11791     -667     
- Misses         40      696     +656     
- Partials       21       49      +28

Impacted Files	Coverage Δ
trio/_subprocess.py	99.3% <100%> (-0.7%)	⬇️
trio/tests/test_subprocess.py	96.38% <100%> (-3.62%)	⬇️
trio/__init__.py	97.22% <100%> (ø)	⬆️
trio/_core/_windows_cffi.py	0% <0%> (-100%)	⬇️
trio/_wait_for_object.py	0% <0%> (-100%)	⬇️
trio/_subprocess_platform/windows.py	0% <0%> (-100%)	⬇️
trio/_windows_pipes.py	0% <0%> (-100%)	⬇️
trio/tests/test_wait_for_object.py	8.27% <0%> (-91.73%)	⬇️
trio/_core/_io_windows.py	0% <0%> (-91.29%)	⬇️
trio/_core/tests/test_windows.py	14.85% <0%> (-85.15%)	⬇️
... and 19 more

[codecov]

Codecov Report

Merging #1113 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1113      +/-   ##
==========================================
+ Coverage   99.51%   99.51%   +<.01%     
==========================================
  Files         102      102              
  Lines       12519    12536      +17     
  Branches      953      953              
==========================================
+ Hits        12458    12475      +17     
  Misses         40       40              
  Partials       21       21

Impacted Files	Coverage Δ
trio/_subprocess.py	100% <100%> (ø)	⬆️
trio/tests/test_subprocess.py	100% <100%> (ø)	⬆️
trio/__init__.py	97.22% <100%> (ø)	⬆️
trio/_highlevel_open_tcp_listeners.py	100% <0%> (ø)	⬆️

[njsmith]

Eh, no brilliant ideas and it's not really worth fussing over for some code we're going to delete after the next release anyway.

For now I scaled back the PR to only making the interface changes, without the actual switch to using a thread, and then in a few months after Process(...) is gone we can fix the actual issue.

[oremanj]

I'd love to be able to represent async with await ... as just async with ...; asyncssh has a decorator that does this. It would probably require us to stop pretending that the awaitable protocol doesn't exist, though, and is in any event a separate change. This looks good; thanks for making the change!