Don't convert to deferreds when not necessary

synapse/util/async.py

@@ -101,7 +101,7 @@ class ObservableDeferred(object):
             return d
         else:
             success, res = self._result
-            return defer.succeed(res) if success else defer.fail(res)
+            return res if success else defer.fail(res)
 
     def observers(self):
         return self._observers

synapse/util/caches/descriptors.py

@@ -341,7 +341,10 @@ class CacheDescriptor(_CacheDescriptorBase):
                 cache.set(cache_key, result_d, callback=invalidate_callback)
                 observer = result_d.observe()
 
-            return logcontext.make_deferred_yieldable(observer)
+            if isinstance(observer, defer.Deferred):
+                return logcontext.make_deferred_yieldable(observer)
+            else:
+                return observer
 
         wrapped.invalidate = cache.invalidate
         wrapped.invalidate_all = cache.invalidate_all

synapse/util/logcontext.py

@@ -315,6 +315,9 @@ def preserve_context_over_deferred(deferred, context=None):
     the deferred follow the synapse logcontext rules: try
     ``make_deferred_yieldable`` instead.
     """
+    if not isinstance(deferred, defer.Deferred):
+        return deferred
+
     if context is None:
         context = LoggingContext.current_context()
     d = _PreservingContextDeferred(context)