Sliding Sync: Avoid fetching left rooms and add back `newly_left` rooms (#17725)

Performance optimization: We can avoid fetching rooms that the user has
left themselves (which could be a significant amount), then only add
back rooms that the user has `newly_left` (left in the token range of an
incremental sync). It's a lot faster to fetch less rooms than fetch them
all and throw them away in most cases. Since the user only leaves a room
(or is state reset out) once in a blue moon, we can avoid a lot of work.

Based on @erikjohnston's branch, erikj/ss_perf


---------

Co-authored-by: Erik Johnston <erik@matrix.org>

1 files changed, 18 insertions, 2 deletions

diff --git a/synapse/storage/databases/main/state.py b/synapse/storage/databases/main/state.py

index ca31122ad3..60312d770d 100644
--- a/synapse/storage/databases/main/state.py
+++ b/synapse/storage/databases/main/state.py
@@ -308,8 +308,24 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
         return create_event
 
     @cached(max_entries=10000)
-    async def get_room_type(self, room_id: str) -> Optional[str]:
-        raise NotImplementedError()
+    async def get_room_type(self, room_id: str) -> Union[Optional[str], Sentinel]:
+        """Fetch room type for given room.
+
+        Since this function is cached, any missing values would be cached as
+        `None`. In order to distinguish between an unencrypted room that has
+        `None` encryption and a room that is unknown to the server where we
+        might want to omit the value (which would make it cached as `None`),
+        instead we use the sentinel value `ROOM_UNKNOWN_SENTINEL`.
+        """
+
+        try:
+            create_event = await self.get_create_event_for_room(room_id)
+            return create_event.content.get(EventContentFields.ROOM_TYPE)
+        except NotFoundError:
+            # We use the sentinel value to distinguish between `None` which is a
+            # valid room type and a room that is unknown to the server so the value
+            # is just unset.
+            return ROOM_UNKNOWN_SENTINEL
 
     @cachedList(cached_method_name="get_room_type", list_name="room_ids")
     async def bulk_get_room_type(