`gevent` execmode hangs in python 3.7.5

The problem only occurs at python3.

Environment:

• OS: Linux faa410b1f09b 4.19.121-linuxkit #1 SMP Thu Jan 21 15:36:34 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
• Python version: Python 3.7.5

Dependencies:

apipkg         1.5
attrs          20.3.0
execnet        1.8.0
gevent         21.1.2
greenlet       1.0.0
iniconfig      1.1.1
packaging      20.9
pip            19.2.3
pluggy         0.13.1
py             1.10.0
pyparsing      2.4.7
pytest         6.2.3
pytest-forked  1.3.0
pytest-xdist   2.2.1
setuptools     41.2.0
toml           0.10.2
zope.event     4.5.0
zope.interface 5.4.0

Command: pytest -d --tx popen//execmodel=gevent test.py

Code:

from gevent import monkey
monkey.patch_all()


def test_assert_0():
    assert 0


def test_assert_1():
    assert 1

In Python 2 environment command runs all tests, reports failure on test_assert_0 and ends. In Python 3 environment test hangs before scanning for tests.

Execnet debug logs:

====================================== test session starts =======================================
platform darwin -- Python 3.8.2, pytest-6.2.3, py-1.10.0, pluggy-0.13.1
rootdir: /Users/changwan.park/Documents/py3_venv
plugins: xdist-2.2.1, forked-1.3.0
gw0 C[13946] gw0 [receiver-thread] RECEIVERTHREAD: starting to run
[13946] gw0 sent <Message CHANNEL_EXEC channel=1 lendata=8631>
[13946] gw0 sent <Message CHANNEL_DATA channel=1 lendata=470>
[13948] creating workergateway on <execnet.gateway_base.Popen2IO object at 0x10dc89bb0>
[13948] gw0-worker [serve] spawning receiver thread
[13948] gw0-worker [serve] joining receiver thread
[13948] gw0-worker waiting for receiver thread to finish
[13948] gw0-worker [receiver-thread] RECEIVERTHREAD: starting to run
[13948] gw0-worker [receiver-thread] received <Message CHANNEL_EXEC channel=1 lendata=8631>
[13948] gw0-worker [receiver-thread] received <Message CHANNEL_DATA channel=1 lendata=470>

Different issue compared with https://github.com/pytest-dev/execnet/issues/75. By comparing execnet debug trace, above example hangs after gw0-worker only receives gw0but does not execute the received command.

After some digging, I found out the hanging occurres since remote_exec hangs. See https://github.com/pytest-dev/execnet/issues/123.

Reproduce the issue with the shown pytest command and test.py under Python 3.7.5, then compare the execnet debug trace with the Python 2 behavior. Start by reading the referenced execnet remote_exec issue and the gevent execmode path; done means the command no longer hangs before test collection and reports the expected failing assertion.