1 files changed, 23 insertions, 4 deletions
diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index 59b4cf1e53..3d62451de9 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -348,7 +348,14 @@ class EventFederationStore(SQLBaseStore):
         # 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)
-        stream_ordering = min(last_change, stream_ordering)
+
+        # 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
+        last_change = max(self._stream_order_on_start, last_change)
+
+        if last_change > self.stream_ordering_month_ago:
+            stream_ordering = min(last_change, stream_ordering)
 
         return self._get_forward_extremeties_for_room(room_id, stream_ordering)
 
@@ -369,7 +376,7 @@ class EventFederationStore(SQLBaseStore):
                 INNER JOIN (
                     SELECT room_id, MAX(stream_ordering) AS stream_ordering
                     FROM stream_ordering_to_exterm
-                    WHERE stream_ordering < ? GROUP BY room_id
+                    WHERE stream_ordering <= ? GROUP BY room_id
                 ) AS rms USING (room_id, stream_ordering)
                 WHERE room_id = ?
         """)
@@ -386,9 +393,21 @@ class EventFederationStore(SQLBaseStore):
 
     def _delete_old_forward_extrem_cache(self):
         def _delete_old_forward_extrem_cache_txn(txn):
+            # Delete entries older than a month, while making sure we don't delete
+            # the only entries for a room.
+            sql = ("""
+                DELETE FROM stream_ordering_to_exterm
+                WHERE
+                (
+                    SELECT max(stream_ordering) AS stream_ordering
+                    FROM stream_ordering_to_exterm
+                    WHERE room_id = stream_ordering_to_exterm.room_id
+                ) > ?
+                AND stream_ordering < ?
+            """)
             txn.execute(
-                "DELETE FROM stream_ordering_to_exterm WHERE stream_ordering < ?",
-                (self.stream_ordering_month_ago,)
+                sql,
+                (self.stream_ordering_month_ago, self.stream_ordering_month_ago,)
             )
         return self.runInteraction(
             "_delete_old_forward_extrem_cache",