Intelligently select extremities used in backfill. (#8349)

Instead of just using the most recent extremities let's pick the
ones that will give us results that the pagination request cares about,
i.e. pick extremities only if they have a smaller depth than the
pagination token.

This is useful when we fail to backfill an extremity, as we no longer
get stuck requesting that same extremity repeatedly.

synapse/storage/databases/main/stream.py

@@ -648,23 +648,20 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         )
         return "t%d-%d" % (row["topological_ordering"], row["stream_ordering"])
 
-    async def get_max_topological_token(self, room_id: str, stream_key: int) -> int:
-        """Get the max topological token in a room before the given stream
+    async def get_current_topological_token(self, room_id: str, stream_key: int) -> int:
+        """Gets the topological token in a room after or at the given stream
         ordering.
 
         Args:
             room_id
             stream_key
-
-        Returns:
-            The maximum topological token.
         """
         sql = (
-            "SELECT coalesce(max(topological_ordering), 0) FROM events"
-            " WHERE room_id = ? AND stream_ordering < ?"
+            "SELECT coalesce(MIN(topological_ordering), 0) FROM events"
+            " WHERE room_id = ? AND stream_ordering >= ?"
         )
         row = await self.db_pool.execute(
-            "get_max_topological_token", None, sql, room_id, stream_key
+            "get_current_topological_token", None, sql, room_id, stream_key
         )
         return row[0][0] if row else 0