Add support metadata in policy.json

* if there is a policy.json in the course dir, read it
* file format is a dict with keys {category}/{url_name}, and values metadata dictionaries
* apply the policy, overwriting keys that are in the xml
* then do metadata inheritance, inheriting any overwritten keys.

* also a management cmd to generate a policy.json from a course dir.

common/lib/xmodule/xmodule/modulestore/xml.py

@@ -1,3 +1,4 @@
+import json
 import logging
 import os
 import re
@@ -149,7 +150,7 @@ class XMLModuleStore(ModuleStoreBase):
         for course_dir in course_dirs:
             self.try_load_course(course_dir)
 
-    def try_load_course(self,course_dir):
+    def try_load_course(self, course_dir):
         '''
         Load a course, keeping track of errors as we go along.
         '''
@@ -170,7 +171,27 @@ class XMLModuleStore(ModuleStoreBase):
         '''
         String representation - for debugging
         '''
-        return '<XMLModuleStore>data_dir=%s, %d courses, %d modules' % (self.data_dir,len(self.courses),len(self.modules))
+        return '<XMLModuleStore>data_dir=%s, %d courses, %d modules' % (
+            self.data_dir, len(self.courses), len(self.modules))
+
+    def load_policy(self, policy_path, tracker):
+        """
+        Attempt to read a course policy from policy_path.  If the file
+        exists, but is invalid, log an error and return {}.
+
+        If the policy loads correctly, returns the deserialized version.
+        """
+        if not os.path.exists(policy_path):
+            return {}
+        try:
+            with open(policy_path) as f:
+                return json.load(f)
+        except (IOError, ValueError) as err:
+            msg = "Error loading course policy from {}".format(policy_path)
+            tracker(msg)
+            log.warning(msg + " " + str(err))
+        return {}
+
 
     def load_course(self, course_dir, tracker):
         """
@@ -214,6 +235,11 @@ class XMLModuleStore(ModuleStoreBase):
             system = ImportSystem(self, org, course, course_dir, tracker)
 
             course_descriptor = system.process_xml(etree.tostring(course_data))
+            policy_path = self.data_dir / course_dir / 'policy.json'
+
+            policy = self.load_policy(policy_path, tracker)
+            XModuleDescriptor.apply_policy(course_descriptor, policy)
+
             # NOTE: The descriptors end up loading somewhat bottom up, which
             # breaks metadata inheritance via get_children().  Instead
             # (actually, in addition to, for now), we do a final inheritance pass

common/lib/xmodule/xmodule/x_module.py

@@ -298,6 +298,14 @@ class XModule(HTMLSnippet):
         return ""
 
 
+def policy_key(location):
+    """
+    Get the key for a location in a policy file.  (Since the policy file is
+    specific to a course, it doesn't need the full location url).
+    """
+    return '{cat}/{name}'.format(cat=location.category, name=location.name)
+
+
 class XModuleDescriptor(Plugin, HTMLSnippet):
     """
     An XModuleDescriptor is a specification for an element of a course. This
@@ -416,6 +424,24 @@ class XModuleDescriptor(Plugin, HTMLSnippet):
         return dict((k,v) for k,v in self.metadata.items()
                     if k not in self._inherited_metadata)
 
+
+    @staticmethod
+    def apply_policy(node, policy):
+        """
+        Given a descriptor, traverse all its descendants and update its metadata
+        with the policy.
+
+        Notes:
+          - this does not propagate inherited metadata.  The caller should
+            call compute_inherited_metadata after applying the policy.
+          - metadata specified in the policy overrides metadata in the xml
+        """
+        k = policy_key(node.location)
+        if k in policy:
+            node.metadata.update(policy[k])
+        for c in node.get_children():
+            XModuleDescriptor.apply_policy(c, policy)
+
     @staticmethod
     def compute_inherited_metadata(node):
         """Given a descriptor, traverse all of its descendants and do metadata