ResponseCache: fix handling of completed results

Turns out that ObservableDeferred.observe doesn't return a deferred if the
result is already completed. Fix handling and improve documentation.

synapse/util/caches/response_cache.py

@@ -14,6 +14,8 @@
 # limitations under the License.
 import logging
 
+from twisted.internet import defer
+
 from synapse.util.async import ObservableDeferred
 from synapse.util.caches import metrics as cache_metrics
 from synapse.util.logcontext import make_deferred_yieldable, run_in_background
@@ -48,15 +50,21 @@ class ResponseCache(object):
     def get(self, key):
         """Look up the given key.
 
-        Returns a deferred which doesn't follow the synapse logcontext rules,
-        so you'll probably want to make_deferred_yieldable it.
+        Can return either a new Deferred (which also doesn't follow the synapse
+        logcontext rules), or, if the request has completed, the actual
+        result. You will probably want to make_deferred_yieldable the result.
+
+        If there is no entry for the key, returns None. It is worth noting that
+        this means there is no way to distinguish a completed result of None
+        from an absent cache entry.
 
         Args:
             key (hashable):
 
         Returns:
-            twisted.internet.defer.Deferred|None: None if there is no entry
-            for this key; otherwise a deferred result.
+            twisted.internet.defer.Deferred|None|E: None if there is no entry
+            for this key; otherwise either a deferred result or the result
+            itself.
         """
         result = self.pending_result_cache.get(key)
         if result is not None:
@@ -73,19 +81,17 @@ class ResponseCache(object):
         you should wrap normal synapse deferreds with
         logcontext.run_in_background).
 
-        Returns a new Deferred which also doesn't follow the synapse logcontext
-        rules, so you will want to make_deferred_yieldable it
-
-        (TODO: before using this more widely, it might make sense to refactor
-        it and get() so that they do the necessary wrapping rather than having
-        to do it everywhere ResponseCache is used.)
+        Can return either a new Deferred (which also doesn't follow the synapse
+        logcontext rules), or, if *deferred* was already complete, the actual
+        result. You will probably want to make_deferred_yieldable the result.
 
         Args:
             key (hashable):
-            deferred (twisted.internet.defer.Deferred):
+            deferred (twisted.internet.defer.Deferred[T):
 
         Returns:
-            twisted.internet.defer.Deferred
+            twisted.internet.defer.Deferred[T]|T: a new deferred, or the actual
+                result.
         """
         result = ObservableDeferred(deferred, consumeErrors=True)
         self.pending_result_cache[key] = result
@@ -144,7 +150,7 @@ class ResponseCache(object):
                         self._name, key)
             d = run_in_background(callback, *args, **kwargs)
             result = self.set(key, d)
-        elif result.called:
+        elif not isinstance(result, defer.Deferred) or result.called:
             logger.info("[%s]: using completed cached result for [%s]",
                         self._name, key)
         else: