pytest stuck after assertion fail

Detailed Description

I'm writing pytest test cases for aiosmtpd, and I'm hitting this problem against master in aio-libs/aiosmtpd

Minimal reproducible bug as follows:

import asyncio
import pytest

from aiosmtpd.controller import Controller
from aiosmtpd.handlers import Sink
from aiosmtpd.smtp import SMTP as Server
from smtplib import SMTP


SERVER_ADDRESS = ("localhost", 8025)


def authenticator(mechanism, login, password):
    if login and login.decode() == 'goodlogin':
        return True
    else:
        return False


class DecodingController(Controller):
    def factory(self):
        return Server(self.handler, decode_data=True, enable_SMTPUTF8=True,
                      auth_require_tls=False, auth_callback=authenticator)


@pytest.fixture
def temp_loop() -> asyncio.AbstractEventLoop:
    default_loop = asyncio.get_event_loop()
    new_loop = asyncio.new_event_loop()
    asyncio.set_event_loop(new_loop)
    #
    yield new_loop
    #
    new_loop.close()
    asyncio.set_event_loop(default_loop)


@pytest.fixture
def decoding_controller(temp_loop):
    controller = DecodingController(Sink, loop=temp_loop)
    controller.start()
    temp_loop.call_later(3.0, temp_loop.stop)
    #
    print("@@@ yielding @@@")
    yield controller
    print("@@@ returning @@@")
    #
    controller.stop()


@pytest.fixture
def client():
    print(">>> entering <<<")
    with SMTP(*SERVER_ADDRESS) as smtp_client:
        yield smtp_client
    print(">>> exited <<<")


@pytest.mark.usefixtures("decoding_controller")
class TestSMTPNieuw:

    def test_data_abortconn(self, client):
        code, _ = client.helo("example.com")
        assert code == 250
        code, _ = client.docmd("MAIL FROM: <alice@example.org>")
        assert code == 250
        code, _ = client.docmd("RCPT TO: <bob@example.org>")
        assert code == 250
        print("###SENDING_DATA###")
        code, _ = client.docmd("DATA")
        assert code == 250  # I purposefully fail here
        print("###ASSERT_SUCCESS###")

Running the above inside PyCharm's Terminal window produces this:

Microsoft Windows [Version 10.0.19041.508]
(c) 2020 Microsoft Corporation. All rights reserved.

(aiosmtpd-10-cp36) C:\Repos\aiosmtpd>pytest -vvvv -s aiosmtpd\tests\test_smtp_problem.py
=========================================== test session starts ===========================================
platform win32 -- Python 3.6.8, pytest-6.1.1, py-1.9.0, pluggy-0.13.1 -- c:\repos\venvs\aiosmtpd-10-cp36\scripts\python.exe
cachedir: .pytest_cache
rootdir: C:\Repos\aiosmtpd
plugins: mock-3.3.1
collected 1 item

aiosmtpd/tests/test_smtp_problem.py::TestSMTPNieuw::test_data_abortconn @@@ yielding @@@
>>> entering <<<
###SENDING_DATA###
FAILED

... then pytest just got stuck there, not responding to anything, I can't even do Ctrl-C and I have to terminate the Terminal process.

pip list

Package            Version
------------------ -------
atomicwrites       1.4.0
atpublic           2.1.1
attrs              20.2.0
colorama           0.4.4
importlib-metadata 2.0.0
iniconfig          1.1.1
packaging          20.4
pip                20.2.4
pluggy             0.13.1
py                 1.9.0
pyparsing          2.4.7
pytest             6.1.2
pytest-mock        3.3.1
setuptools         50.3.2
six                1.15.0
toml               0.10.2
typing-extensions  3.7.4.3
zipp               3.4.0

(I actually only installed 3 packages: atpublic, pytest, and pytest-mock; the other packages are the dependencies of those three.)

pytest & OS versions

(aiosmtpd-11-cp36) C:\Repos\aiosmtpd>pytest -V
pytest 6.1.2

(aiosmtpd-11-cp36) C:\Repos\aiosmtpd>ver
Microsoft Windows [Version 10.0.19041.508]

(aiosmtpd-11-cp36) C:\Repos\aiosmtpd>python -V
Python 3.6.8

Minimal example

See "Detailed Description" above

• a detailed description of the bug or problem you are having
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

Reproduce the failure from the minimal example in aiosmtpd/tests/test_smtp_problem.py using pytest 6.1.2 on Windows and Python 3.6.8. Start by examining the failed test's fixture teardown, especially decoding_controller and temp_loop; done means pytest exits cleanly after the assertion failure and remains interruptible.