Make ObservableDeferred.observe() always return deferred.

This makes it easier to use in an async/await world.

Also fixes a bug where cache descriptors would occaisonally return a raw
value rather than a deferred.

1 files changed, 2 insertions, 5 deletions

diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index 7659eaeb42..fd75ba27ad 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -86,11 +86,8 @@ class ObservableDeferred(object):
 
         deferred.addCallbacks(callback, errback)
 
-    def observe(self):
+    def observe(self) -> defer.Deferred:
         """Observe the underlying deferred.
-
-        Can return either a deferred if the underlying deferred is still pending
-        (or has failed), or the actual value. Callers may need to use maybeDeferred.
         """
         if not self._result:
             d = defer.Deferred()
@@ -105,7 +102,7 @@ class ObservableDeferred(object):
             return d
         else:
             success, res = self._result
-            return res if success else defer.fail(res)
+            return defer.succeed(res) if success else defer.fail(res)
 
     def observers(self):
         return self._observers