🐛 FIX: Cleanup process event hooks (#200)

On Process close/cleanup event hooks are removed,
in part to not persist cyclic dependencies of hooks <-> Process.
Once a process is closed, it will also not raise an Exception
if a hook tries to unregister itself (but has already been removed).

plumpy/base/state_machine.py

@@ -286,6 +286,9 @@ def add_state_event_callback(self, hook: Hashable, callback: EVENT_CALLBACK_TYPE
         self._event_callbacks.setdefault(hook, []).append(callback)
 
     def remove_state_event_callback(self, hook: Hashable, callback: EVENT_CALLBACK_TYPE) -> None:
+        if getattr(self, '_closed', False):
+            # if the process is closed, then all callbacks have already been removed
+            return None
         try:
             self._event_callbacks[hook].remove(callback)
         except (KeyError, ValueError):

plumpy/processes.py

@@ -317,17 +317,16 @@ def try_killing(future: futures.Future) -> None:
 
     def _setup_event_hooks(self) -> None:
         """Set the event hooks to process, when it is created or loaded(recreated)."""
-        self.add_state_event_callback(
-            state_machine.StateEventHook.ENTERING_STATE,
-            lambda _s, _h, state: self.on_entering(cast(process_states.State, state))
-        )
-        self.add_state_event_callback(
-            state_machine.StateEventHook.ENTERED_STATE,
-            lambda _s, _h, from_state: self.on_entered(cast(Optional[process_states.State], from_state))
-        )
-        self.add_state_event_callback(
-            state_machine.StateEventHook.EXITING_STATE, lambda _s, _h, _state: self.on_exiting()
-        )
+        event_hooks = {
+            state_machine.StateEventHook.ENTERING_STATE:
+            lambda _s, _h, state: self.on_entering(cast(process_states.State, state)),
+            state_machine.StateEventHook.ENTERED_STATE:
+            lambda _s, _h, from_state: self.on_entered(cast(Optional[process_states.State], from_state)),
+            state_machine.StateEventHook.EXITING_STATE:
+            lambda _s, _h, _state: self.on_exiting()
+        }
+        for hook, callback in event_hooks.items():
+            self.add_state_event_callback(hook, callback)
 
     @property
     def creation_time(self) -> Optional[float]:
@@ -845,6 +844,7 @@ def on_close(self) -> None:
                     self.logger.exception('Exception calling cleanup method %s', cleanup)
             self._cleanups = None
         finally:
+            self._event_callbacks = {}
             self._closed = True
 
     def _fire_event(self, evt: Callable[..., Any], *args: Any, **kwargs: Any) -> None: