[1.9.x] Fixed #25389 -- Fixed pickling a SimpleLazyObject wrapping a model.

Pickling a `SimpleLazyObject` wrapping a model did not work correctly; in
particular it did not add the `_django_version` attribute added in 42736ac8.
Now it will handle this and other custom `__reduce__` methods correctly.

Backport of 35355a4f from master

django/utils/functional.py

@@ -3,7 +3,6 @@ import operator
 from functools import total_ordering, wraps
 
 from django.utils import six
-from django.utils.six.moves import copyreg
 
 
 # You can't trivially replace this with `functools.partial` because this binds
@@ -247,32 +246,30 @@ class LazyObject(object):
         raise NotImplementedError('subclasses of LazyObject must provide a _setup() method')
 
     # Because we have messed with __class__ below, we confuse pickle as to what
-    # class we are pickling. It also appears to stop __reduce__ from being
-    # called. So, we define __getstate__ in a way that cooperates with the way
-    # that pickle interprets this class.  This fails when the wrapped class is
-    # a builtin, but it is better than nothing.
-    def __getstate__(self):
+    # class we are pickling. We're going to have to initialize the wrapped
+    # object to successfully pickle it, so we might as well just pickle the
+    # wrapped object since they're supposed to act the same way.
+    #
+    # Unfortunately, if we try to simply act like the wrapped object, the ruse
+    # will break down when pickle gets our id(). Thus we end up with pickle
+    # thinking, in effect, that we are a distinct object from the wrapped
+    # object, but with the same __dict__. This can cause problems (see #25389).
+    #
+    # So instead, we define our own __reduce__ method and custom unpickler. We
+    # pickle the wrapped object as the unpickler's argument, so that pickle
+    # will pickle it normally, and then the unpickler simply returns its
+    # argument.
+    def __reduce__(self):
         if self._wrapped is empty:
             self._setup()
-        return self._wrapped.__dict__
-
-    # Python 3 will call __reduce__ when pickling; this method is needed
-    # to serialize and deserialize correctly.
-    @classmethod
-    def __newobj__(cls, *args):
-        return cls.__new__(cls, *args)
-
-    def __reduce_ex__(self, proto):
-        if proto >= 2:
-            # On Py3, since the default protocol is 3, pickle uses the
-            # ``__newobj__`` method (& more efficient opcodes) for writing.
-            return (self.__newobj__, (self.__class__,), self.__getstate__())
-        else:
-            # On Py2, the default protocol is 0 (for back-compat) & the above
-            # code fails miserably (see regression test). Instead, we return
-            # exactly what's returned if there's no ``__reduce__`` method at
-            # all.
-            return (copyreg._reconstructor, (self.__class__, object, None), self.__getstate__())
+        return (unpickle_lazyobject, (self._wrapped,))
+
+    # We have to explicitly override __getstate__ so that older versions of
+    # pickle don't try to pickle the __dict__ (which in the case of a
+    # SimpleLazyObject may contain a lambda). The value will end up being
+    # ignored by our __reduce__ and custom unpickler.
+    def __getstate__(self):
+        return {}
 
     def __deepcopy__(self, memo):
         if self._wrapped is empty:
@@ -311,6 +308,15 @@ class LazyObject(object):
     __contains__ = new_method_proxy(operator.contains)
 
 
+def unpickle_lazyobject(wrapped):
+    """
+    Used to unpickle lazy objects. Just return its argument, which will be the
+    wrapped object.
+    """
+    return wrapped
+unpickle_lazyobject.__safe_for_unpickling__ = True
+
+
 # Workaround for http://bugs.python.org/issue12370
 _super = super
 

docs/releases/1.8.5.txt

@@ -56,3 +56,5 @@ Bugfixes
 * Fixed incorrect queries with multiple many-to-many fields on a model with the
   same 'to' model and with ``related_name`` set to '+' (:ticket:`24505`,
   :ticket:`25486`).
+
+* Fixed pickling a ``SimpleLazyObject`` wrapping a model (:ticket:`25389`).

tests/utils_tests/models.py

@@ -11,3 +11,7 @@ class Category(models.Model):
 class Thing(models.Model):
     name = models.CharField(max_length=100)
     category = models.ForeignKey(Category, models.CASCADE)
+
+
+class CategoryInfo(models.Model):
+    category = models.OneToOneField(Category, models.CASCADE)

tests/utils_tests/test_lazyobject.py

@@ -3,11 +3,14 @@ from __future__ import unicode_literals
 import copy
 import pickle
 import sys
+import warnings
 from unittest import TestCase
 
 from django.utils import six
 from django.utils.functional import LazyObject, SimpleLazyObject, empty
 
+from .models import Category, CategoryInfo
+
 
 class Foo(object):
     """
@@ -284,3 +287,92 @@ class SimpleLazyObjectTestCase(LazyObjectTestCase):
         self.assertNotIn(6, lazy_set)
         self.assertEqual(len(lazy_list), 5)
         self.assertEqual(len(lazy_set), 4)
+
+
+class BaseBaz(object):
+    """
+    A base class with a funky __reduce__ method, meant to simulate the
+    __reduce__ method of Model, which sets self._django_version.
+    """
+    def __init__(self):
+        self.baz = 'wrong'
+
+    def __reduce__(self):
+        self.baz = 'right'
+        return super(BaseBaz, self).__reduce__()
+
+    def __eq__(self, other):
+        if self.__class__ != other.__class__:
+            return False
+        for attr in ['bar', 'baz', 'quux']:
+            if hasattr(self, attr) != hasattr(other, attr):
+                return False
+            elif getattr(self, attr, None) != getattr(other, attr, None):
+                return False
+        return True
+
+
+class Baz(BaseBaz):
+    """
+    A class that inherits from BaseBaz and has its own __reduce_ex__ method.
+    """
+    def __init__(self, bar):
+        self.bar = bar
+        super(Baz, self).__init__()
+
+    def __reduce_ex__(self, proto):
+        self.quux = 'quux'
+        return super(Baz, self).__reduce_ex__(proto)
+
+
+class BazProxy(Baz):
+    """
+    A class that acts as a proxy for Baz. It does some scary mucking about with
+    dicts, which simulates some crazy things that people might do with
+    e.g. proxy models.
+    """
+    def __init__(self, baz):
+        self.__dict__ = baz.__dict__
+        self._baz = baz
+        super(BaseBaz, self).__init__()
+
+
+class SimpleLazyObjectPickleTestCase(TestCase):
+    """
+    Regression test for pickling a SimpleLazyObject wrapping a model (#25389).
+    Also covers other classes with a custom __reduce__ method.
+    """
+    def test_pickle_with_reduce(self):
+        """
+        Test in a fairly synthetic setting.
+        """
+        # Test every pickle protocol available
+        for protocol in range(pickle.HIGHEST_PROTOCOL + 1):
+            lazy_objs = [
+                SimpleLazyObject(lambda: BaseBaz()),
+                SimpleLazyObject(lambda: Baz(1)),
+                SimpleLazyObject(lambda: BazProxy(Baz(2))),
+            ]
+            for obj in lazy_objs:
+                pickled = pickle.dumps(obj, protocol)
+                unpickled = pickle.loads(pickled)
+                self.assertEqual(unpickled, obj)
+                self.assertEqual(unpickled.baz, 'right')
+
+    def test_pickle_model(self):
+        """
+        Test on an actual model, based on the report in #25426.
+        """
+        category = Category.objects.create(name="thing1")
+        CategoryInfo.objects.create(category=category)
+        # Test every pickle protocol available
+        for protocol in range(pickle.HIGHEST_PROTOCOL + 1):
+            lazy_category = SimpleLazyObject(lambda: category)
+            # Test both if we accessed a field on the model and if we didn't.
+            lazy_category.categoryinfo
+            lazy_category_2 = SimpleLazyObject(lambda: category)
+            with warnings.catch_warnings(record=True) as recorded:
+                self.assertEqual(pickle.loads(pickle.dumps(lazy_category, protocol)), category)
+                self.assertEqual(pickle.loads(pickle.dumps(lazy_category_2, protocol)), category)
+                # Assert that there were no warnings.
+                self.assertEqual(len(recorded), 0)