Sometimes when a socket transport under ProactorEventLoop is writing while the peer closes the connection, asyncio logs an AssertionError. 

Attached is a script that fairly reliably reproduces the behavior on my computer.

This is caused by _ProactorBasePipeTransport._force_close() being called between two invocations of _ProactorBaseWritePipeTransport._loop_writing(), where the latter call asserts self._write_fut has not changed after being set by the former call.

Adding Victor as he's been helping with asyncio/proactor lately.

> Sometimes when a socket transport under ProactorEventLoop is writing while the peer closes the connection, asyncio logs an AssertionError. 

Would you mind to add an example of such error in this issue?

No problem. Running the attached test script on latest master, Windows 10 1803, several errors like this are logged:

Exception in callback _ProactorBaseWritePipeTransport._loop_writing(<_OverlappedF...events.py:479>)
handle: <Handle _ProactorBaseWritePipeTransport._loop_writing(<_OverlappedF...events.py:479>) created at %USERPROFILE%\source\repos\cpython\lib\asyncio\proactor_events.py:373>
source_traceback: Object created at (most recent call last):
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\base_events.py", line 555, in run_until_complete
    self.run_forever()
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\base_events.py", line 523, in run_forever
    self._run_once()
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\base_events.py", line 1750, in _run_once
    handle._run()
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "..\..\..\..\Documents\python\test_proactor_force_close.py", line 8, in server_callback
    writer.write(b'hello')
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\streams.py", line 305, in write
    self._transport.write(data)
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\proactor_events.py", line 334, in write
    self._loop_writing(data=bytes(data))
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\proactor_events.py", line 373, in _loop_writing
    self._write_fut.add_done_callback(self._loop_writing)
Traceback (most recent call last):
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "%USERPROFILE%\source\repos\cpython\lib\asyncio\proactor_events.py", line 346, in _loop_writing
    assert f is self._write_fut
AssertionError

As an aside, I'm wondering whether it makes sense to add a blanket "assert exception handler has not been called" condition to ProactorEventLoop's tests, or even other asyncio tests. It looks like ProactorEventLoop is more prone to dropping exceptions on the floor than SelectorEventLoop.

New changeset 9045199c5aaeac9b52537581be127d999b5944ee by Andrew Svetlov (twisteroid ambassador) in branch 'master':
bpo-33833: Fix ProactorSocketTransport AssertionError (#7893)
https://github.com/python/cpython/commit/9045199c5aaeac9b52537581be127d999b5944ee

New changeset d5c75be55504fae1ff510eed66cddbd27bfbdbe2 by Miss Islington (bot) in branch '3.7':
bpo-33833: Fix ProactorSocketTransport AssertionError (GH-7893)
https://github.com/python/cpython/commit/d5c75be55504fae1ff510eed66cddbd27bfbdbe2

Why not fixing Python 3.6? It is not affected?