Add 'event' result to 'context' endpoint

... because the context isn't much use without the event.

synapse/handlers/room.py

@@ -864,28 +864,39 @@ class RoomContextHandler(BaseHandler):
                 (excluding state).
 
         Returns:
-            dict
+            dict, or None if the event isn't found
         """
         before_limit = math.floor(limit/2.)
         after_limit = limit - before_limit
 
         now_token = yield self.hs.get_event_sources().get_current_token()
 
+        def filter_evts(events):
+            return self._filter_events_for_client(
+                user.to_string(),
+                events,
+                is_guest=is_guest)
+
+        event = yield self.store.get_event(event_id, get_prev_content=True,
+                                           allow_none=True)
+        if not event:
+            defer.returnValue(None)
+            return
+
+        filtered = yield(filter_evts([event]))
+        if not filtered:
+            raise AuthError(
+                403,
+                "You don't have permission to access that event."
+            )
+
         results = yield self.store.get_events_around(
             room_id, event_id, before_limit, after_limit
         )
 
-        results["events_before"] = yield self._filter_events_for_client(
-            user.to_string(),
-            results["events_before"],
-            is_guest=is_guest,
-        )
-
-        results["events_after"] = yield self._filter_events_for_client(
-            user.to_string(),
-            results["events_after"],
-            is_guest=is_guest,
-        )
+        results["events_before"] = yield filter_evts(results["events_before"])
+        results["events_after"] = yield filter_evts(results["events_after"])
+        results["event"] = event
 
         if results["events_after"]:
             last_event_id = results["events_after"][-1].event_id

synapse/rest/client/v1/room.py

@@ -402,10 +402,16 @@ class RoomEventContext(ClientV1RestServlet):
             user, room_id, event_id, limit, is_guest
         )
 
+        if not results:
+            raise SynapseError(
+                404, "Event not found.", errcode=Codes.NOT_FOUND
+            )
+
         time_now = self.clock.time_msec()
         results["events_before"] = [
             serialize_event(event, time_now) for event in results["events_before"]
         ]
+        results["event"] = serialize_event(results["event"], time_now)
         results["events_after"] = [
             serialize_event(event, time_now) for event in results["events_after"]
         ]