Allow chained auth functions to chain off built-in auth functions.

ckan/authz.py

@@ -102,13 +102,19 @@ def _build(self):
                     fetched_auth_functions[name] = auth_function
 
         for name, func_list in chained_auth_functions.iteritems():
-            if name not in fetched_auth_functions:
+            if name not in fetched_auth_functions and\
+                    name not in self._functions:
                 raise Exception('The auth %r is not found for chained auth' % (
                     name))
             # create the chain of functions in the correct order
             for func in reversed(func_list):
-                prev_func = fetched_auth_functions[name]
-                fetched_auth_functions[name] = functools.partial(func, prev_func)
+                if name in fetched_auth_functions:
+                    prev_func = fetched_auth_functions[name]
+                else:
+                    # fallback to chaining off the builtin auth function
+                    prev_func = self._functions[name]
+                fetched_auth_functions[name] =\
+                    functools.partial(func, prev_func)
 
         # Use the updated ones in preference to the originals.
         self._functions.update(fetched_auth_functions)