Exclude outliers in on_backfill_request

When we are processing a `/backfill` request from a remote server, exclude any
outliers from consideration early on. We can't return outliers anyway (since we
don't know the state at the outlier), and filtering them out earlier means that
we won't attempt to calulate the state for them.

changelog.d/12314.misc

@@ -0,0 +1 @@
+Avoid trying to calculate the state at outlier events.

synapse/storage/databases/main/event_federation.py

@@ -1073,9 +1073,15 @@ def _get_connected_prev_event_backfill_results_txn(
             /* Get the depth and stream_ordering of the prev_event_id from the events table */
             INNER JOIN events
             ON prev_event_id = events.event_id
+
+            /* exclude outliers from the results (we don't have the state, so cannot
+             * verify if the requesting server can see them).
+             */
+            WHERE NOT events.outlier
+
             /* Look for an edge which matches the given event_id */
-            WHERE event_edges.event_id = ?
-            AND event_edges.is_state = ?
+            AND event_edges.event_id = ? AND NOT event_edges.is_state
+
             /* Because we can have many events at the same depth,
             * we want to also tie-break and sort on stream_ordering */
             ORDER BY depth DESC, stream_ordering DESC
@@ -1084,7 +1090,7 @@ def _get_connected_prev_event_backfill_results_txn(
 
         txn.execute(
             connected_prev_event_query,
-            (event_id, False, limit),
+            (event_id, limit),
         )
         return [
             BackfillQueueNavigationItem(