Fixed #18590 - Reverted Python 2.4 workaround for Model pickling

Revert of 08d521efa0. Refs #10547, #12121.
Thanks Michal Petrucha for the report.

django/db/models/base.py

@@ -16,7 +16,7 @@ from django.db.models.fields.related import (ManyToOneRel,
 from django.db import (router, transaction, DatabaseError,
     DEFAULT_DB_ALIAS)
 from django.db.models.query import Q
-from django.db.models.query_utils import DeferredAttribute
+from django.db.models.query_utils import DeferredAttribute, deferred_class_factory
 from django.db.models.deletion import Collector
 from django.db.models.options import Options
 from django.db.models import signals
@@ -400,25 +400,16 @@ class Model(object):
         need to do things manually, as they're dynamically created classes and
         only module-level classes can be pickled by the default path.
         """
+        if not self._deferred:
+            return super(Model, self).__reduce__()
         data = self.__dict__
-        model = self.__class__
-        # The obvious thing to do here is to invoke super().__reduce__()
-        # for the non-deferred case. Don't do that.
-        # On Python 2.4, there is something weird with __reduce__,
-        # and as a result, the super call will cause an infinite recursion.
-        # See #10547 and #12121.
         defers = []
-        if self._deferred:
-            from django.db.models.query_utils import deferred_class_factory
-            factory = deferred_class_factory
         for field in self._meta.fields:
             if isinstance(self.__class__.__dict__.get(field.attname),
                     DeferredAttribute):
                 defers.append(field.attname)
         model = self._meta.proxy_for_model
-        else:
+        return (model_unpickle, (model, defers), data)
-            factory = simple_class_factory
-        return (model_unpickle, (model, defers, factory), data)
 
     def _get_pk_val(self, meta=None):
         if not meta:
@@ -926,20 +917,11 @@ def get_absolute_url(opts, func, self, *args, **kwargs):
 class Empty(object):
     pass
 
-def simple_class_factory(model, attrs):
+def model_unpickle(model, attrs):
-    """Used to unpickle Models without deferred fields.
-
-    We need to do this the hard way, rather than just using
-    the default __reduce__ implementation, because of a
-    __deepcopy__ problem in Python 2.4
-    """
-    return model
-
-def model_unpickle(model, attrs, factory):
     """
     Used to unpickle Model subclasses with deferred fields.
     """
-    cls = factory(model, attrs)
+    cls = deferred_class_factory(model, attrs)
     return cls.__new__(cls)
 model_unpickle.__safe_for_unpickle__ = True