[New-bugs-announce] [issue28232] asyncio: wrap_future() doesn't handle cancellation correctly

[STINNER Victor]

New submission from STINNER Victor:

I noticed the following warning in test_asyncio (see above). It understand that when the asyncio Future ("destination" in _chain_future) is cancelled, _call_check_cancel() calls source_loop.call_soon_threadsafe(source.cancel) where source is a concurrent.futures.Future object.

Then futures.Future.cancel() calls self._invoke_callbacks() which calls _call_set_state() of _chain_future(). _call_set_state() calls dest_loop.call_soon_threadsafe(_set_state, destination, source) but at this point, the event loop is closed. Morever, destination (the asyncio future) is already cancelled, but it doesn't make sense to try to copy the state: source and destination are already cancelled.

I suggest to modify _call_set_state() to do nothing if destination is already cancelled:

    def _call_set_state(source):
        if destination.cancelled():
            return
        (...)

*But* I see that futures.Future has a set_running_or_notify_cancel() method which seems to be important to call. Maybe we should call it in this case?

    def _call_set_state(source):
        if destination.cancelled():
            source.set_running_or_notify_cancel()
            return
        (...)

--

The warning:


http://buildbot.python.org/all/builders/AMD64%20Windows8%203.6/builds/43/steps/test/logs/stdio

test_sock_sendall (test.test_asyncio.test_selector_events.BaseSelectorEventLoopTests) ... exception calling callback for <Future at 0xec840d1e28 state=finished returned NoneType>
Traceback (most recent call last):
  File "D:\buildarea\3.6.bolen-windows8\build\lib\concurrent\futures\_base.py", line 297, in _invoke_callbacks
    callback(self)
  File "D:\buildarea\3.6.bolen-windows8\build\lib\asyncio\futures.py", line 462, in _call_set_state
    dest_loop.call_soon_threadsafe(_set_state, destination, source)
  File "D:\buildarea\3.6.bolen-windows8\build\lib\asyncio\base_events.py", line 603, in call_soon_threadsafe
    handle = self._call_soon(callback, args)
  File "D:\buildarea\3.6.bolen-windows8\build\lib\asyncio\base_events.py", line 577, in _call_soon
    self._check_closed()
  File "D:\buildarea\3.6.bolen-windows8\build\lib\asyncio\base_events.py", line 356, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed
ok

----------
components: asyncio
messages: 277107
nosy: gvanrossum, haypo, yselivanov
priority: normal
severity: normal
status: open
title: asyncio: wrap_future() doesn't handle cancellation correctly
versions: Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28232>
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