asyncio.create_subprocess_exec with IO redirection hangs if no global loop set

[rutsky]

If I set global event loop to None (asyncio.set_event_loop(None)) and explicitly pass everywhere event loop, asyncio.create_subprocess_exec with I/O redirection will hang.

In following example proc.communicate() will never return:

import asyncio.subprocess

async def f(loop):
    proc = await asyncio.create_subprocess_exec(
        'echo',
        stdout=asyncio.subprocess.PIPE,
        loop=loop)

    await proc.communicate()

loop = asyncio.get_event_loop()

asyncio.set_event_loop(None)

date = loop.run_until_complete(f(loop))
loop.close()

With PYTHONASYNCIODEBUG=x:

Traceback (most recent call last):
  File "asyncio_subprocess_hangs_wo_global_loop.py", line 17, in <module>
    date = loop.run_until_complete(f(loop))
  File "/usr/lib/python3.5/asyncio/base_events.py", line 387, in run_until_complete
    return future.result()
  File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
    raise self._exception
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "asyncio_subprocess_hangs_wo_global_loop.py", line 9, in f
    loop=loop)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/subprocess.py", line 212, in create_subprocess_exec
    stderr=stderr, **kwds)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1079, in subprocess_exec
    bufsize, **kwargs)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 187, in _make_subprocess_transport
    self._child_watcher_callback, transp)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 808, in add_child_handler
    self._do_waitpid(pid)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 841, in _do_waitpid
    if self._loop.get_debug():
AttributeError: 'NoneType' object has no attribute 'get_debug'
Task was destroyed but it is pending!
source_traceback: Object created at (most recent call last):
  File "asyncio_subprocess_hangs_wo_global_loop.py", line 17, in <module>
    date = loop.run_until_complete(f(loop))
  File "/usr/lib/python3.5/asyncio/base_events.py", line 375, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 345, in run_forever
    self._run_once()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1304, in _run_once
    handle._run()
  File "/usr/lib/python3.5/asyncio/events.py", line 125, in _run
    self._callback(*self._args)
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "asyncio_subprocess_hangs_wo_global_loop.py", line 9, in f
    loop=loop)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/subprocess.py", line 212, in create_subprocess_exec
    stderr=stderr, **kwds)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1079, in subprocess_exec
    bufsize, **kwargs)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 184, in _make_subprocess_transport
    **kwargs)
  File "/usr/lib/python3.5/asyncio/base_subprocess.py", line 56, in __init__
    self._loop.create_task(self._connect_pipes(waiter))
task: <Task pending coro=<BaseSubprocessTransport._connect_pipes() running at /usr/lib/python3.5/asyncio/base_subprocess.py:171> wait_for=<Future pending cb=[Task._wakeup()] created at /usr/lib/python3.5/asyncio/base_events.py:252> created at /usr/lib/python3.5/asyncio/base_subprocess.py:56>
Future exception was never retrieved
future: <Future finished exception=RuntimeError('Event loop is closed',) created at /usr/lib/python3.5/asyncio/base_events.py:252>
source_traceback: Object created at (most recent call last):
  File "asyncio_subprocess_hangs_wo_global_loop.py", line 17, in <module>
    date = loop.run_until_complete(f(loop))
  File "/usr/lib/python3.5/asyncio/base_events.py", line 375, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 345, in run_forever
    self._run_once()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1304, in _run_once
    handle._run()
  File "/usr/lib/python3.5/asyncio/events.py", line 125, in _run
    self._callback(*self._args)
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "asyncio_subprocess_hangs_wo_global_loop.py", line 9, in f
    loop=loop)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/subprocess.py", line 212, in create_subprocess_exec
    stderr=stderr, **kwds)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1079, in subprocess_exec
    bufsize, **kwargs)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 105, in __next__
    return self.gen.send(None)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 180, in _make_subprocess_transport
    waiter = self.create_future()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 252, in create_future
    return futures.Future(loop=self)
Traceback (most recent call last):
  File "/usr/lib/python3.5/asyncio/base_events.py", line 989, in connect_read_pipe
    yield from waiter
GeneratorExit

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.5/asyncio/base_subprocess.py", line 171, in _connect_pipes
    proc.stdout)
  File "/usr/lib/python3.5/asyncio/coroutines.py", line 127, in close
    return self.gen.close()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 991, in connect_read_pipe
    transport.close()
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 376, in close
    self._close(None)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 404, in _close
    self._loop.call_soon(self._call_connection_lost, exc)
  File "/usr/lib/python3.5/asyncio/base_events.py", line 497, in call_soon
    handle = self._call_soon(callback, args)
  File "/usr/lib/python3.5/asyncio/base_events.py", line 506, in _call_soon
    self._check_closed()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 334, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

Tested on Ubuntu 16.04, with Python 3.5.2.

[vxgmichel]

A similar issue has been reported (#390), and the hanging problem has been fixed in PR #391. However, you still need to attach the event loop to the child watcher explicitly in order to manage subprocesses without relying on the asyncio policy to provide the event loop (specific to Unix):

asyncio.set_event_loop(None)
loop = asyncio.new_event_loop()
asyncio.get_child_watcher().attach_loop(loop)

Also related: #421, see this comment.

[rutsky]

@vxgmichel thanks for the information!

Attaching loop to the child watcher helps.

So this is already fixed and will be included in the next Python 3.5 release?