Fixed #22426 -- Added support old-style d.c.messages format.

Forward ported code from 1.5 that adds backwards compatibility with legacy message length.
See commit 9e7183073f for details.

Thanks to Ofir Ovadia for the initial patch.

django/contrib/messages/storage/cookie.py

@@ -33,6 +33,9 @@ class MessageDecoder(json.JSONDecoder):
     def process_messages(self, obj):
         if isinstance(obj, list) and obj:
             if obj[0] == MessageEncoder.message_key:
+                if len(obj) == 3:
+                    # Compatibility with previously-encoded messages
+                    return Message(*obj[1:])
                 if obj[1]:
                     obj[3] = mark_safe(obj[3])
                 return Message(*obj[2:])

django/contrib/messages/tests/test_cookie.py

@@ -153,3 +153,25 @@ class CookieTest(BaseTests, TestCase):
             encode_decode(mark_safe("<b>Hello Django!</b>")), SafeData)
         self.assertNotIsInstance(
             encode_decode("<b>Hello Django!</b>"), SafeData)
+
+    def test_pre_1_5_message_format(self):
+        """
+        For ticket #22426. Tests whether messages that were set in the cookie
+        before the addition of is_safedata are decoded correctly.
+        """
+
+        # Encode the messages using the current encoder.
+        messages = [Message(constants.INFO, 'message %s') for x in range(5)]
+        encoder = MessageEncoder(separators=(',', ':'))
+        encoded_messages = encoder.encode(messages)
+
+        # Remove the is_safedata flag from the messages in order to imitate
+        # the behavior of before 1.5 (monkey patching).
+        encoded_messages = json.loads(encoded_messages)
+        for obj in encoded_messages:
+            obj.pop(1)
+        encoded_messages = json.dumps(encoded_messages, separators=(',', ':'))
+
+        # Decode the messages in the old format (without is_safedata)
+        decoded_messages = json.loads(encoded_messages, cls=MessageDecoder)
+        self.assertEqual(messages, decoded_messages)