fix: Break reference cycle between Actor and ActorRef

Actor and ActorRef both had references to each other, creating a cycle
that blocked instant memory freeing since the actor's reference count
never reached zero.

This changes the ActorRef reference to the actor to use a weakref, so
that the ActorRef doesn't contribute to the Actor's reference count,
enabling instant freeing of the memory.

Fixes #249, #250

Author: jodal

src/pykka/_proxy.py

@@ -138,8 +138,11 @@ def __init__(
         if not actor_ref.is_alive():
             msg = f"{actor_ref} not found"
             raise ActorDeadError(msg)
+        if (actor := actor_ref._actor_weakref()) is None:  # noqa: SLF001
+            msg = f"{actor_ref}'s actor weakref has been deallocated"
+            raise ActorDeadError(msg)
         self.actor_ref = actor_ref
-        self._actor = actor_ref._actor  # noqa: SLF001
+        self._actor = actor
         self._attr_path = attr_path or ()
         self._known_attrs = introspect_attrs(root=self._actor, proxy=self)
         self._actor_proxies = {}

src/pykka/_ref.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import weakref
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -51,7 +52,7 @@ def __init__(
         self,
         actor: A,
     ) -> None:
-        self._actor = actor
+        self._actor_weakref = weakref.ref(actor)
         self.actor_class = actor.__class__
         self.actor_urn = actor.actor_urn
         self.actor_inbox = actor.actor_inbox