Make send_eof a standard part of the Stream interface

[njsmith]

The only reason HalfCloseableStream existed as a separate interface was because SSLStream couldn't support send_eof. But TLS 1.3 made it possible to support send_eof! So now the static split doesn't make sense. Instead, we move send_eof into the Stream interface (though it can still fail at runtime with NotImplementedError), and get rid of HalfCloseableStream. (Fixes: gh-823.)

This also includes a fix for StapledStream.send_eof, that was revealed by the new enhanced check_two_way_stream. (Fixes: gh-1180).

[oremanj]

This looks great overall. One yapf nit to fix, and one backwards compatibility concern: adding send_eof() as an abstract method of Stream will break any user-provided Stream implementation that doesn't define it. Another approach with better properties there:

• make it non-abstract and have the default implementation raise NotImplementedError
• plus use __init_subclass__ to raise a TrioDeprecationWarning when defining a subclass that doesn't override it
• could use __new__ to warn on construction of such subclasses on 3.5 which doesn't call __init_subclass__, or could decide not to worry about that

I'm also OK leaving this incompatibility in, but it should probably be called out more explicitly in the newsfragments in that case.

[njsmith]

@oremanj okay, fine, you're right :-)

I already figured out how to do this kind of deprecation properly while looking at #636, so might as well do it here too...

[codecov[bot]]

Codecov Report

Merging #1181 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1181      +/-   ##
==========================================
+ Coverage   99.57%   99.57%   +<.01%     
==========================================
  Files         106      106              
  Lines       12836    12928      +92     
  Branches      985      996      +11     
==========================================
+ Hits        12781    12873      +92     
  Misses         35       35              
  Partials       20       20

Impacted Files	Coverage Δ
trio/abc.py	100% <ø> (ø)	:arrow_up:
trio/tests/test_abc.py	100% <100%> (ø)	:arrow_up:
trio/_ssl.py	100% <100%> (ø)	:arrow_up:
trio/testing/_check_streams.py	99.38% <100%> (+0.04%)	:arrow_up:
trio/_highlevel_generic.py	100% <100%> (ø)	:arrow_up:
trio/_abc.py	100% <100%> (ø)	:arrow_up:
trio/tests/test_highlevel_socket.py	100% <100%> (ø)	:arrow_up:
trio/tests/test_ssl.py	100% <100%> (ø)	:arrow_up:
trio/_highlevel_socket.py	100% <100%> (ø)	:arrow_up:
trio/_unix_pipes.py	100% <100%> (ø)	:arrow_up:
... and 2 more

[njsmith]

Still need to fix up some coverage.

[njsmith]

OK, I think everything's been addressed now.

[oremanj]

Sorry for dropping this. I took another look just now and I'm happy with it, except that the version numbers of the deprecations need to be increased from 0.13.0 to 0.15.0.