Assert that a fetch->cancelled->resumed->fetch cycle is impossible

distributed/worker.py

@@ -2329,7 +2329,14 @@ def transition_resumed_fetch(
         self, ts: TaskState, *, stimulus_id: str
     ) -> RecsInstrs:
         """See Worker._transition_from_resumed"""
-        return self._transition_from_resumed(ts, "fetch", stimulus_id=stimulus_id)
+        recs, instructions = self._transition_from_resumed(
+            ts, "fetch", stimulus_id=stimulus_id
+        )
+        # This would only be possible in a fetch->cancelled->fetch loop, but there's
+        # no such transition from fetch which sets the state to cancelled.
+        # Thus, there's no need to call _ensure_communicating.
+        assert ts.state != "fetch"
+        return recs, instructions
 
     def transition_resumed_missing(
         self, ts: TaskState, *, stimulus_id: str