fix crash in peer eval xmodule

As documented in Jira LMS-806:

Steps to reproduce:
* as one user, for a peer evaluated problem, enter something that needs
  peer grading
* as another user, log in and fire up the peer grading panel. You should
  see something that needs attention, a yellow exclamation badge
* clicking on the "do grading" panel will fail with a 500, stacktrace
  attached.
* it's possible that the question where this is being done must first have
  professor calibration done in order for peers to be able to actually get
  work to grade.

Don Mitchell debugged this with me and saw some problems in the ways
that the peer_evaluation xmodule code was accessing dates. Instead of
using the normal lms accessors, by going straight into the _module_data
it would be bypassing some important code (caching?). Fixing this to be
more standard did the trick.

common/lib/xmodule/xmodule/peer_grading_module.py

@@ -12,7 +12,7 @@ from xmodule.modulestore.django import modulestore
 from xmodule.modulestore.exceptions import ItemNotFoundError
 from .timeinfo import TimeInfo
 from xblock.core import Dict, String, Scope, Boolean, Integer, Float
-from xmodule.fields import Date
+from xmodule.fields import Date, Timedelta
 
 from xmodule.open_ended_grading_classes.peer_grading_service import PeerGradingService, GradingServiceError, MockPeerGradingService
 from open_ended_grading_classes import combined_open_ended_rubric
@@ -47,9 +47,8 @@ class PeerGradingFields(object):
         help="Due date that should be displayed.",
         default=None,
         scope=Scope.settings)
-    grace_period_string = String(
+    graceperiod = Timedelta(
         help="Amount of grace to give on the due date.",
-        default=None,
         scope=Scope.settings
     )
     student_data_for_location = Dict(
@@ -105,12 +104,12 @@ class PeerGradingModule(PeerGradingFields, XModule):
                 log.error("Linked location {0} for peer grading module {1} does not exist".format(
                     self.link_to_location, self.location))
                 raise
-            due_date = self.linked_problem._model_data.get('due', None)
+            due_date = self.linked_problem.lms.due
             if due_date:
-                self._model_data['due'] = due_date
+                self.lms.due = due_date
 
         try:
-            self.timeinfo = TimeInfo(self.due, self.grace_period_string)
+            self.timeinfo = TimeInfo(self.due, self.graceperiod)
         except Exception:
             log.error("Error parsing due date information in location {0}".format(self.location))
             raise
@@ -533,10 +532,10 @@ class PeerGradingModule(PeerGradingFields, XModule):
             problem_location = problem['location']
             descriptor = _find_corresponding_module_for_location(problem_location)
             if descriptor:
-                problem['due'] = descriptor._model_data.get('due', None)
-                grace_period_string = descriptor._model_data.get('graceperiod', None)
+                problem['due'] = descriptor.lms.due
+                grace_period = descriptor.lms.graceperiod
                 try:
-                    problem_timeinfo = TimeInfo(problem['due'], grace_period_string)
+                    problem_timeinfo = TimeInfo(problem['due'], grace_period)
                 except:
                     log.error("Malformed due date or grace period string for location {0}".format(problem_location))
                     raise
@@ -629,5 +628,5 @@ class PeerGradingDescriptor(PeerGradingFields, RawDescriptor):
     @property
     def non_editable_metadata_fields(self):
         non_editable_fields = super(PeerGradingDescriptor, self).non_editable_metadata_fields
-        non_editable_fields.extend([PeerGradingFields.due, PeerGradingFields.grace_period_string])
+        non_editable_fields.extend([PeerGradingFields.due, PeerGradingFields.graceperiod])
         return non_editable_fields

common/lib/xmodule/xmodule/timeinfo.py

@@ -14,20 +14,23 @@ class TimeInfo(object):
 
     """
     _delta_standin = Timedelta()
-    def __init__(self, due_date, grace_period_string):
+    def __init__(self, due_date, grace_period_string_or_timedelta):
         if due_date is not None:
             self.display_due_date = due_date
 
         else:
             self.display_due_date = None
 
-        if grace_period_string is not None and self.display_due_date:
-            try:
-                self.grace_period = TimeInfo._delta_standin.from_json(grace_period_string)
-                self.close_date = self.display_due_date + self.grace_period
-            except:
-                log.error("Error parsing the grace period {0}".format(grace_period_string))
-                raise
+        if grace_period_string_or_timedelta is not None and self.display_due_date:
+            if isinstance(grace_period_string_or_timedelta, basestring):
+                try:
+                    self.grace_period = TimeInfo._delta_standin.from_json(grace_period_string_or_timedelta)
+                except:
+                    log.error("Error parsing the grace period {0}".format(grace_period_string_or_timedelta))
+                    raise
+            else:
+                self.grace_period = grace_period_string_or_timedelta
+            self.close_date = self.display_due_date + self.grace_period
         else:
             self.grace_period = None
             self.close_date = self.display_due_date