asyncioreactor almost certainly broken on Windows with Python 3.8

[twisted-trac]

	@cdunklau reported
Trac ID	trac#9766
Type	defect
Created	2020-02-18 00:46:12Z

AsyncioSelectorReactor uses get_event_loop by default, which is nice and convenient and good... except when the policy makes it return a non-SelectorEventLoop.

In bpo-34687, Python 3.8 made ProactorEventLoop the default for Windows , and that link also states:

ProactorEventLoop has the following limitations: The loop.add_reader() and loop.add_writer() methods are not supported.

So when you do try to use AsyncioSelectorReactor on py3.8+win, e.g. in the tests for pytest-twisted, you get a rather befuddling error:

<snip>
  File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\asyncioreactor.py", line 320, in install
    reactor = AsyncioSelectorReactor(eventloop)
  File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\asyncioreactor.py", line 69, in __init__
    super().__init__()
  File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\base.py", line 571, in __init__
    self.installWaker()
  File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\posixbase.py", line 286, in installWaker
    self.addReader(self.waker)
  File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\asyncioreactor.py", line 151, in addReader
    self._asyncioEventloop.add_reader(fd, callWithLogger, reader,
  File "C:\Python38-x64\Lib\asyncio\events.py", line 501, in add_reader
    raise NotImplementedError
NotImplementedError

So, what to do? I hesitate to call the whole situation a Twisted defect, but I do think Twisted should not break in the face of something that should be detectable. I don't know really know the right way to detect this case... the first thought I had was "just typecheck against asyncio.BaseSelectorEventLoop"... which would work, but is maybe icky.

Searchable metadata

trac-id__9766 9766
type__defect defect
reporter__cdunklau cdunklau
priority__normal normal
milestone__Python_3_8_Support Python 3.8 Support
branch__ 
branch_author__ 
status__closed closed
resolution__fixed fixed
component__core core
keywords__None None
time__1581986772283411 1581986772283411
changetime__1600072597809031 1600072597809031
version__None None
owner__Craig_Rodrigues__rodrigc_____ Craig Rodrigues <rodrigc@...>

[twisted-trac]

	@njsmith commented

Tornado ran into the same issue and started a discussion upstream a few months ago: https://bugs.python.org/issue37373

[twisted-trac]

	@rodrigc commented

#1338

[twisted-trac]

	@altendky set owner to @rodrigc

Review:
#1338 (review)

Just a partial first pass as changes were added mid-review

[twisted-trac]

	@rodrigc removed owner

[twisted-trac]

	@altendky set owner to @rodrigc

Review: #1338 (comment)

[twisted-trac]

	@rodrigc removed owner

Approved here: #1338 (review)

[twisted-trac]

	@rodrigc set owner to @rodrigc @rodrigc set status to closed

In changeset d8203ca

#!CommitTicketReference repository="" revision="d8203ca6e51705bd6246e063722fcd8596b7cbd3"
Merge pull request #1338 from twisted/9766-rodrigc-asyncio-selecteventorloop

Author: rodrigc
Reviewer: twm
Fixes: ticket:9766 ticket:9809

Fix AsyncioSelectorReactor on Windows Python 3.8+