1 files changed, 16 insertions, 1 deletions
diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index ee88c61954..256e50dc20 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -281,15 +281,30 @@ class EventFederationStore(SQLBaseStore):
         )
 
     def get_forward_extremeties_for_room(self, room_id, stream_ordering):
+        """For a given room_id and stream_ordering, return the forward
+        extremeties of the room at that point in "time".
+
+        Throws a StoreError if we have since purged the index for
+        stream_orderings from that point.
+
+        Args:
+            room_id (str):
+            stream_ordering (int):
+
+        Returns:
+            deferred, which resolves to a list of event_ids
+        """
         # We want to make the cache more effective, so we clamp to the last
         # change before the given ordering.
         last_change = self._events_stream_cache.get_max_pos_of_last_change(room_id)
 
         # We don't always have a full stream_to_exterm_id table, e.g. after
         # the upgrade that introduced it, so we make sure we never ask for a
-        # try and pin to a stream_ordering from before a restart
+        # stream_ordering from before a restart
         last_change = max(self._stream_order_on_start, last_change)
 
+        # provided the last_change is recent enough, we now clamp the requested
+        # stream_ordering to it.
         if last_change > self.stream_ordering_month_ago:
             stream_ordering = min(last_change, stream_ordering)