Limit number of extremeties in backfill request

This works around a bug where if we make a backfill request with too
many extremeties it causes the request URI to be too long.

synapse/handlers/federation.py

@@ -274,7 +274,7 @@ class FederationHandler(BaseHandler):
 
     @log_function
     @defer.inlineCallbacks
-    def backfill(self, dest, room_id, limit, extremities=[]):
+    def backfill(self, dest, room_id, limit, extremities):
         """ Trigger a backfill request to `dest` for the given `room_id`
 
         This will attempt to get more events from the remote. This may return
@@ -284,9 +284,6 @@ class FederationHandler(BaseHandler):
         if dest == self.server_name:
             raise SynapseError(400, "Can't backfill from self.")
 
-        if not extremities:
-            extremities = yield self.store.get_oldest_events_in_room(room_id)
-
         events = yield self.replication_layer.backfill(
             dest,
             room_id,
@@ -455,6 +452,10 @@ class FederationHandler(BaseHandler):
         )
         max_depth = sorted_extremeties_tuple[0][1]
 
+        # We don't want to specify too many extremities as it causes the backfill
+        # request URI to be too long.
+        extremities = dict(sorted_extremeties_tuple[:5])
+
         if current_depth > max_depth:
             logger.debug(
                 "Not backfilling as we don't need to. %d < %d",