Here is the minimal example code:

https://gist.github.com/achimnol/965a6aecf7b1f96207abf11469b68965

Just run this code using "python -m pytest -s test.py" to see what happens.
(My setup uses Python 3.6.4 and pytest 3.3.2 on macOS High Sierra 10.13.2)

I tried the same logic using synchornous APIs such as threading.Thread / time.sleep instead of loop.create_task / asyncio.sleep, it raised "ValueError: generator already executing" when tried to close the generator.