Event loop is closed error

[scott]

I got an error with a flow on Prefect 2 that I’m not sure how to debug. I haven’t found any related issues in Issues · PrefectHQ/prefect · GitHub or Slack, etc.

The flow failed, and the stacktrace looks like:

Exception ignored in: <function BaseSubprocessTransport.__del__ at 0x7f9cf6c191b0>
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/asyncio/base_subprocess.py", line 126, in __del__
    self.close()
  File "/usr/local/lib/python3.10/asyncio/base_subprocess.py", line 104, in close
    proto.pipe.close()
  File "/usr/local/lib/python3.10/asyncio/unix_events.py", line 547, in close
    self._close(None)
  File "/usr/local/lib/python3.10/asyncio/unix_events.py", line 571, in _close
    self._loop.call_soon(self._call_connection_lost, exc)
  File "/usr/local/lib/python3.10/asyncio/base_events.py", line 753, in call_soon
    self._check_closed()
  File "/usr/local/lib/python3.10/asyncio/base_events.py", line 515, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed
20:26:13.266 | INFO    | prefect.infrastructure.process - Process 'electric-serval' exited cleanly.

Could this error have too many underlying issues to figure out, or does this point to some known issue?

Our setup:
Running 3 Prefect agents on a GCP VM, and the code that the flows kick off run inside Docker containers. Flows are kicked off for now in the Cloud UI.

[anna_geller]

maybe this can help?

Best practices for using async in Prefect 2.0

When writing async flows/tasks, all the standard rules of async python apply. https://docs.python.org/3/library/asyncio-task.html It’s a good practice to run any blocking code in a separate thread, e.g. running time.sleep in a separate worker thread here works well: from prefect.utilities.asyncutils import run_sync_in_worker_thread async def async_func(): if random.random() > 0.3: run_sync_in_worker_thread(time.sleep, 1) await asyncio.sleep(random.randint(1, 5)) return 1 …

[scott]

Thanks, but this flow wasn’t using any async code. Sorry I forgot to mention that. Have you seen this kind of error before? Perhaps it’s related to the prefect python library?

[anna_geller]

there’s not enough context to help, we would need a minimal reproducible example, context of how you run it incl. prefect version

[scott]

I’d love to but since I don’t know what caused the error I can’t make a minimal rep eg.

[anna_geller]

I understand, but we can’t help solve a bug if we can’t reproduce it. Perhaps try doing the same what you did that led to that error and try to generalize and build a small example you can share in a GitHub issue?