Aiohttp: pytest-aiohttp + pytest-xdist cause RuntimeError: set_wakeup_fd só funciona na thread principal

Longa história curta

Combinando pytest-aiohttp e pytest-xdist um erro é disparado às vezes do fixture loop .

Deixe-me saber se você acha que o problema está mais no lado pytest (-xdist).

Comportamento esperado

Sem erro.

Comportamento real

=================================================================================================================== ERRORS ====================================================================================================================
_____________________________________________________________________________________________________ ERROR at setup of test_foo[pyloop] ______________________________________________________________________________________________________
[gw6] linux -- Python 3.7.0 /home/merlin/workspace/pytest-test/.venv/bin/python3.7

loop_factory = <class 'asyncio.unix_events._UnixDefaultEventLoopPolicy'>, fast = False, loop_debug = False

    @pytest.fixture
    def loop(loop_factory, fast, loop_debug):
        """Return an instance of the event loop."""
        policy = loop_factory()
        asyncio.set_event_loop_policy(policy)
>       with loop_context(fast=fast) as _loop:

.venv/lib/python3.7/site-packages/aiohttp/pytest_plugin.py:205:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/usr/local/lib/python3.7/contextlib.py:112: in __enter__
    return next(self.gen)
.venv/lib/python3.7/site-packages/aiohttp/test_utils.py:404: in loop_context
    loop = setup_test_loop(loop_factory)
.venv/lib/python3.7/site-packages/aiohttp/test_utils.py:427: in setup_test_loop
    watcher.attach_loop(loop)
/usr/local/lib/python3.7/asyncio/unix_events.py:868: in attach_loop
    loop.add_signal_handler(signal.SIGCHLD, self._sig_chld)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <_UnixSelectorEventLoop running=False closed=False debug=False>, sig = <Signals.SIGCHLD: 17>, callback = <bound method BaseChildWatcher._sig_chld of <asyncio.unix_events.SafeChildWatcher object at 0x7fbf10c9b048>>, args = ()

    def add_signal_handler(self, sig, callback, *args):
        """Add a handler for a signal.  UNIX only.

        Raise ValueError if the signal number is invalid or uncatchable.
        Raise RuntimeError if there is a problem setting up the handler.
        """
        if (coroutines.iscoroutine(callback) or
                coroutines.iscoroutinefunction(callback)):
            raise TypeError("coroutines cannot be used "
                            "with add_signal_handler()")
        self._check_signal(sig)
        self._check_closed()
        try:
            # set_wakeup_fd() raises ValueError if this is not the
            # main thread.  By calling it early we ensure that an
            # event loop running in another thread cannot add a signal
            # handler.
            signal.set_wakeup_fd(self._csock.fileno())
        except (ValueError, OSError) as exc:
>           raise RuntimeError(str(exc))
E           RuntimeError: set_wakeup_fd only works in main thread

/usr/local/lib/python3.7/asyncio/unix_events.py:94: RuntimeError
===================================================================================================== 10 passed, 1 error in 0.93 seconds ======================================================================================================

Passos para reproduzir

Copiei o mesmo arquivo de teste 10 vezes (apenas para ter alguns testes para trabalhar).

async def test_foo(loop):
    assert 42 == 42

Eu uso o bash para executar o pytest várias vezes até que ele falhe:

while pytest tests/ -n auto -v; do :; done

Não sei por que, mas a sinalização detalhada parece afetá-lo: -v

Seu ambiente

Testado em 3.6 e 3.7.
CPU de 8 núcleos

Python 3.6.7
Python 3.7.0b5

aiohttp==3.4.4
pytest==4.0.1
pytest-aiohttp==0.3.0
pytest-xdist==1.25.0