This is tightly related with #6867 and dask/dask#9330.

There is a deadlock which is triggered by this code path:

which in turn triggers:

The deadlock should be reproducible as follows:

1. handle_stimulus(ComputeTaskEvent(key="x")
   ts.state=executing; create asyncio task for Worker.execute
2. handle_stimulus(FreeKeysEvent(keys=["x"])
   ts.state=cancelled
3. await asyncio.sleep(0)
   Worker.execute runs and returns AlreadyCancelledEvent.
   This causes the _handle_stimulus_from_task callback to be appended to the end of the event loop.
   However, the test suite is before that in the event loop:
4. handle_stimulus(ComputeTaskEvent(key="x")
   ts.state=resumed
5. await ... (anything that releases the event loop)
   This runs _handle_stimulus_from_task,
   which runs _handle_already_cancelled,
   which returns {ts: "released"},
   which triggers the (resumed, released) transition,
   which sends the task to cancelled state, while the scheduler thinks it's running.

@fjetter @gjoseph92 my head is spinning.