Improve /sync performance of when passing filters with empty arrays. (#14786)

This has two related changes:

* It enables fast-path processing for an empty filter (`[]`) which was
  previously only used for wildcard not-filters (`["*"]`).
* It special cases a `/sync` filter with no-rooms to skip all room
  processing, previously we would partially skip processing, but would
  generally still calculate intermediate values for each room which were
  then unused.

Future changes might consider further optimizations:

* Skip calculating per-room account data when all rooms are filtered (currently
  this is thrown away).
* Make similar improvements to other endpoints which support filters.

changelog.d/14786.feature

@@ -0,0 +1 @@
+Improve performance of `/sync` when filtering all rooms, message types, or senders.

synapse/api/filtering.py

@@ -283,6 +283,9 @@ class FilterCollection:
             await self._room_filter.filter(events)
         )
 
+    def blocks_all_rooms(self) -> bool:
+        return self._room_filter.filters_all_rooms()
+
     def blocks_all_presence(self) -> bool:
         return (
             self._presence_filter.filters_all_types()
@@ -351,13 +354,13 @@ class Filter:
             self.not_rel_types = filter_json.get("org.matrix.msc3874.not_rel_types", [])
 
     def filters_all_types(self) -> bool:
-        return "*" in self.not_types
+        return self.types == [] or "*" in self.not_types
 
     def filters_all_senders(self) -> bool:
-        return "*" in self.not_senders
+        return self.senders == [] or "*" in self.not_senders
 
     def filters_all_rooms(self) -> bool:
-        return "*" in self.not_rooms
+        return self.rooms == [] or "*" in self.not_rooms
 
     def _check(self, event: FilterEvent) -> bool:
         """Checks whether the filter matches the given event.
@@ -450,8 +453,8 @@ class Filter:
             if any(map(match_func, disallowed_values)):
                 return False
 
-            # Other the event does not match at least one of the allowed values,
-            # reject it.
+            # Otherwise if the event does not match at least one of the allowed
+            # values, reject it.
             allowed_values = getattr(self, name)
             if allowed_values is not None:
                 if not any(map(match_func, allowed_values)):

synapse/handlers/search.py

@@ -275,7 +275,7 @@ class SearchHandler:
         )
         room_ids = {r.room_id for r in rooms}
 
-        # If doing a subset of all rooms seearch, check if any of the rooms
+        # If doing a subset of all rooms search, check if any of the rooms
         # are from an upgraded room, and search their contents as well
         if search_filter.rooms:
             historical_room_ids: List[str] = []

synapse/handlers/sync.py

@@ -1403,11 +1403,14 @@ class SyncHandler:
 
         logger.debug("Fetching room data")
 
-        res = await self._generate_sync_entry_for_rooms(
+        (
+            newly_joined_rooms,
+            newly_joined_or_invited_or_knocked_users,
+            newly_left_rooms,
+            newly_left_users,
+        ) = await self._generate_sync_entry_for_rooms(
             sync_result_builder, account_data_by_room
         )
-        newly_joined_rooms, newly_joined_or_invited_or_knocked_users, _, _ = res
-        _, _, newly_left_rooms, newly_left_users = res
 
         block_all_presence_data = (
             since_token is None and sync_config.filter_collection.blocks_all_presence()
@@ -1789,6 +1792,11 @@ class SyncHandler:
             - newly_left_rooms
             - newly_left_users
         """
+
+        # If the request doesn't care about rooms then nothing to do!
+        if sync_result_builder.sync_config.filter_collection.blocks_all_rooms():
+            return set(), set(), set(), set()
+
         since_token = sync_result_builder.since_token
 
         # 1. Start by fetching all ephemeral events in rooms we've joined (if required).