Make new masquerading code compatible to existing session data.

The new masquerading code introduced in PR #8775 adds a new attribute to
CourseMasquerade objects stored in the user's session.  When users who have
active masquerading configuration access instances with the new code, and
AttributeError occurs when trying to access the attribute user_name.

This fix ensures that CourseMasquerade objects receive all required attributes
when they are unpickled from the session.

lms/djangoapps/courseware/masquerade.py

@@ -34,12 +34,25 @@ class CourseMasquerade(object):
     Masquerade settings for a particular course.
     """
     def __init__(self, course_key, role='student', user_partition_id=None, group_id=None, user_name=None):
+        # All parameters to this function must be named identically to the corresponding attribute.
+        # If you remove or rename an attribute, also update the __setstate__() method to migrate
+        # old data from users' sessions.
         self.course_key = course_key
         self.role = role
         self.user_partition_id = user_partition_id
         self.group_id = group_id
         self.user_name = user_name
 
+    def __setstate__(self, state):
+        """
+        Ensure that all attributes are initialised when unpickling CourseMasquerade objects.
+
+        Users might still have CourseMasquerade objects from older versions of the code in their
+        session.  These old objects might not have all attributes set, possibly resulting in
+        AttributeErrors.
+        """
+        self.__init__(**state)
+
 
 @require_POST
 @login_required

lms/djangoapps/courseware/tests/test_masquerade.py

@@ -2,6 +2,7 @@
 Unit tests for masquerade.
 """
 import json
+import pickle
 from mock import patch
 from nose.plugins.attrib import attr
 from datetime import datetime
@@ -12,6 +13,7 @@ from django.utils.timezone import UTC
 
 from capa.tests.response_xml_factory import OptionResponseXMLFactory
 from courseware.masquerade import (
+    CourseMasquerade,
     MasqueradingKeyValueStore,
     handle_ajax,
     setup_masquerade,
@@ -394,3 +396,19 @@ class MasqueradingKeyValueStoreTest(TestCase):
                 self.kvs.get(key)
 
         self.assertEqual(self.kvs.get('c'), 'OpenCraft')
+
+
+class CourseMasqueradeTest(TestCase):
+    """
+    Unit tests for the CourseMasquerade class.
+    """
+    def test_unpickling_sets_all_attributes(self):
+        """
+        Make sure that old CourseMasquerade objects receive missing attributes when unpickled from
+        the session.
+        """
+        cmasq = CourseMasquerade(7)
+        del cmasq.user_name
+        pickled_cmasq = pickle.dumps(cmasq)
+        unpickled_cmasq = pickle.loads(pickled_cmasq)
+        self.assertEqual(unpickled_cmasq.user_name, None)