Refactor UI auth implementation

Instead of returning False when auth is incomplete, throw an exception which
can be caught with a wrapper.

synapse/rest/client/v2_alpha/_base.py

@@ -15,12 +15,13 @@
 
 """This module contains base REST classes for constructing client v1 servlets.
 """
-
-from synapse.api.urls import CLIENT_V2_ALPHA_PREFIX
+import logging
 import re
 
-import logging
+from twisted.internet import defer
 
+from synapse.api.errors import InteractiveAuthIncompleteError
+from synapse.api.urls import CLIENT_V2_ALPHA_PREFIX
 
 logger = logging.getLogger(__name__)
 
@@ -57,3 +58,37 @@ def set_timeline_upper_limit(filter_json, filter_timeline_limit):
         filter_json['room']['timeline']["limit"] = min(
             filter_json['room']['timeline']['limit'],
             filter_timeline_limit)
+
+
+def interactive_auth_handler(orig):
+    """Wraps an on_POST method to handle InteractiveAuthIncompleteErrors
+
+    Takes a on_POST method which returns a deferred (errcode, body) response
+    and adds exception handling to turn a InteractiveAuthIncompleteError into
+    a 401 response.
+
+    Normal usage is:
+
+    @interactive_auth_handler
+    @defer.inlineCallbacks
+    def on_POST(self, request):
+        # ...
+        yield self.auth_handler.check_auth
+            """
+    def wrapped(*args, **kwargs):
+        res = defer.maybeDeferred(orig, *args, **kwargs)
+        res.addErrback(_catch_incomplete_interactive_auth)
+        return res
+    return wrapped
+
+
+def _catch_incomplete_interactive_auth(f):
+    """helper for interactive_auth_handler
+
+    Catches InteractiveAuthIncompleteErrors and turns them into 401 responses
+
+    Args:
+        f (failure.Failure):
+    """
+    f.trap(InteractiveAuthIncompleteError)
+    return 401, f.value.result