Use a chain cover index to efficiently calculate auth chain difference

synapse/storage/databases/main/room.py

@@ -1166,6 +1166,37 @@ def set_room_is_public(self, room_id, is_public):
         # It's overridden by RoomStore for the synapse master.
         raise NotImplementedError()
 
+    async def has_auth_chain_index(self, room_id: str) -> bool:
+        """Check if the room has (or can have) a chain cover index.
+
+        Defaults to True if we don't have an entry in `rooms` table nor any
+        events for the room.
+        """
+
+        has_auth_chain_index = await self.db_pool.simple_select_one_onecol(
+            table="rooms",
+            keyvalues={"room_id": room_id},
+            retcol="has_auth_chain_index",
+            desc="has_auth_chain_index",
+            allow_none=True,
+        )
+
+        if has_auth_chain_index:
+            return True
+
+        # It's possible that we already have events for the room in our DB
+        # without a corresponding room entry. If we do then we don't want to
+        # mark the room as having an auth chain cover index.
+        max_ordering = await self.db_pool.simple_select_one_onecol(
+            table="events",
+            keyvalues={"room_id": room_id},
+            retcol="MAX(stream_ordering)",
+            allow_none=True,
+            desc="upsert_room_on_join",
+        )
+
+        return max_ordering is None
+
 
 class RoomStore(RoomBackgroundUpdateStore, RoomWorkerStore, SearchStore):
     def __init__(self, database: DatabasePool, db_conn, hs):
@@ -1182,14 +1213,7 @@ async def upsert_room_on_join(self, room_id: str, room_version: RoomVersion):
         # It's possible that we already have events for the room in our DB
         # without a corresponding room entry. If we do then we don't want to
         # mark the room as having an auth chain cover index.
-        max_ordering = await self.db_pool.simple_select_one_onecol(
-            table="events",
-            keyvalues={"room_id": room_id},
-            retcol="MAX(stream_ordering)",
-            allow_none=True,
-            desc="upsert_room_on_join",
-        )
-        has_auth_chain_index = max_ordering is None
+        has_auth_chain_index = await self.has_auth_chain_index(room_id)
 
         await self.db_pool.simple_upsert(
             desc="upsert_room_on_join",
@@ -1267,14 +1291,7 @@ async def maybe_store_room_on_outlier_membership(
         # It's possible that we already have events for the room in our DB
         # without a corresponding room entry. If we do then we don't want to
         # mark the room as having an auth chain cover index.
-        max_ordering = await self.db_pool.simple_select_one_onecol(
-            table="events",
-            keyvalues={"room_id": room_id},
-            retcol="MAX(stream_ordering)",
-            allow_none=True,
-            desc="maybe_store_room_on_outlier_membership",
-        )
-        has_auth_chain_index = max_ordering is None
+        has_auth_chain_index = await self.has_auth_chain_index(room_id)
 
         await self.db_pool.simple_upsert(
             desc="maybe_store_room_on_outlier_membership",