[FIX] registry: use a weak dictionary for model_cache to avoid memory leaks

The class attribute `model_cache` refers to model classes, which refer to their
own registry.  This cache potentially keeps all past registries alive!

odoo/models.py

@@ -2817,7 +2817,7 @@ class BaseModel(object):
             self._add_magic_fields()
             cls._proper_fields = set(cls._fields)
 
-            cls.pool.model_cache[cls._model_cache_key] = cls
+        cls.pool.model_cache[cls._model_cache_key] = cls
 
         # 2. add custom fields
         self._add_manual_fields(partial)

odoo/modules/registry.py

@@ -7,6 +7,7 @@
 from collections import Mapping, defaultdict, deque
 from contextlib import closing
 from operator import attrgetter
+from weakref import WeakValueDictionary
 import logging
 import os
 import threading
@@ -30,6 +31,9 @@ class Registry(Mapping):
     _lock = threading.RLock()
     _saved_lock = None
 
+    # a cache for model classes, indexed by their base classes
+    model_cache = WeakValueDictionary()
+
     @lazy_classproperty
     def registries(cls):
         """ A mapping from database names to registries. """
@@ -177,12 +181,6 @@ class Registry(Mapping):
         """ Add or replace a model in the registry."""
         self.models[model_name] = model
 
-    @lazy_classproperty
-    def model_cache(cls):
-        """ A cache for model classes, indexed by their base classes. """
-        # we cache 256 classes per registry on average
-        return LRU(cls.registries.count * 256)
-
     @lazy_property
     def field_sequence(self):
         """ Return a function mapping a field to an integer. The value of a