[1.6.x] 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.

Backport of f286721f7f from master

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

@@ -152,3 +152,25 @@ 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)