classification
Title: Closing async generator while it is running does not raise an exception
Type: behavior Stage: patch review
Components: asyncio Versions: Python 3.7, Python 3.6
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: achimnol, asvetlov, njs, yselivanov
Priority: normal Keywords: patch

Created on 2018-01-10 09:31 by achimnol, last changed 2018-01-14 17:21 by Joongi Kim.

Pull Requests
URL Status Linked Edit
PR 5182 closed Joongi Kim, 2018-01-14 17:21
Messages (2)
msg309760 - (view) Author: Joongi Kim (achimnol) * Date: 2018-01-10 09:39
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.
msg309783 - (view) Author: Nathaniel Smith (njs) * (Python committer) Date: 2018-01-10 22:05
It looks like Python's tracking the "running" state of async generators wrong: we should have ag_running set to True when we enter asend/athrow/aclose and False when we exit, but instead it's being toggled back and forth on each *inner* send/throw on the individual coroutines.

Here's a minimal reproducer (using some random recent checkout of master):

>>> async def f():
...     await asyncio.sleep(1)
...     yield
... 
>>> ag = f()
>>> asend_coro = ag.asend(None)
>>> fut = asend_coro.send(None)
# Logically, ag.asend is still running, waiting for that sleep to
# finish, but we have lost track:
>>> ag.ag_running
False
# We can start another call to asend() going simultaneously
>>> fut.set_result(None)
>>> send_coro2 = ag.asend(None)
>>> send_coro2.send(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

It looks like async_generator did handle this case correctly, but didn't have a test case. I just added one: https://github.com/njsmith/async_generator/commit/339fc6309aa6c96244e79b517db0b98ba0ccfb2a
History
Date User Action Args
2018-01-14 17:21:48Joongi Kimsetkeywords: + patch
stage: patch review
pull_requests: + pull_request5035
2018-01-10 22:05:09njssetmessages: + msg309783
versions: + Python 3.7
2018-01-10 09:39:22achimnolsetmessages: + msg309760
2018-01-10 09:31:53achimnolcreate