TypeError: an integer is required (got type ConcurrencySlotAcquisitionError) when canceling a flow run

[yesheneve]

Hi Prefect Team,
We are using prefect[aws]==3.0.2.

1. We created a deployment with concurrency_limit = 1. The deployment is within a work queue with concurrency_limit=20
2. We started a flow run, but the run did not start successfully and its status is AwaitingConcurrencySlot. This is weird because there are no other runs running for that deployment and work queue. I checked the concurrency of that deployment and its Active Slots is 0 and Slot Decay per Second is 0.
3. We canceled that flow run on prefect UI by clicking the cancel button. But the flow run did not get canceled and stuck in canceling status. The log is like this

Encountered exception while killing process for flow run '991d83c8-7c00-478e-a661-e1182ea5a664'. Flow run may not be cancelled.
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/prefect/runner/runner.py", line 813, in _cancel_run
    await self._kill_process(pid)
  File "/usr/local/lib/python3.9/site-packages/prefect/runner/runner.py", line 675, in _kill_process
    os.kill(pid, signal.SIGTERM)
TypeError: an integer is required (got type ConcurrencySlotAcquisitionError)

Could you please help us on solving this issue? We wish to be able to start and stop flow run with concurrency_limit=1 successfully. Thanks so much!

[desertaxle]

Hey @yesheneve! I think upgrading to 3.1.0 or higher may solve your issue. The concurrency limiting implementation for deployments has been updated since 3.0.2. Give upgrading a try and let me know if you run into any other issues!

[yesheneve]

Version:             3.2.12
API version:         0.8.4
Python version:      3.11.11
Git commit:          826eb1a7
Built:               Mon, Mar 10, 2025 4:36 PM
OS/Arch:             linux/x86_64
Profile:             ephemeral
Server type:         ephemeral
Pydantic version:    2.10.6
Server:
  Database:          AWS RDS Postgres
  Postgres version:  16.4
Integrations:
  prefect-aws:       0.5.5

We have upgraded the prefect version as shown above. The same issue still happens.

1. we create a deployment with concurrency as one.
2. we start two runs for that deployment within a short time.
3. The first run is marked as completed the second run gets stuck in the AwaitingConcurrencySlot status
4. The Active Slots is shown to be 1 after the first run is marked as completed.