Improve perf of sync device lists (#17216)

Re-introduces #17191, and includes #17197 and #17214

The basic idea is to stop calling `get_rooms_for_user` everywhere, and
instead use the table `device_lists_changes_in_room`.

Commits reviewable one-by-one.

1 files changed, 7 insertions, 31 deletions

diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py

index d3d40e8682..b7917a99d6 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1886,38 +1886,14 @@ class SyncHandler:
 
         # Step 1a, check for changes in devices of users we share a room
         # with
-        #
-        # We do this in two different ways depending on what we have cached.
-        # If we already have a list of all the user that have changed since
-        # the last sync then it's likely more efficient to compare the rooms
-        # they're in with the rooms the syncing user is in.
-        #
-        # If we don't have that info cached then we get all the users that
-        # share a room with our user and check if those users have changed.
-        cache_result = self.store.get_cached_device_list_changes(
-            since_token.device_list_key
-        )
-        if cache_result.hit:
-            changed_users = cache_result.entities
-
-            result = await self.store.get_rooms_for_users(changed_users)
-
-            for changed_user_id, entries in result.items():
-                # Check if the changed user shares any rooms with the user,
-                # or if the changed user is the syncing user (as we always
-                # want to include device list updates of their own devices).
-                if user_id == changed_user_id or any(
-                    rid in joined_room_ids for rid in entries
-                ):
-                    users_that_have_changed.add(changed_user_id)
-        else:
-            users_that_have_changed = (
-                await self._device_handler.get_device_changes_in_shared_rooms(
-                    user_id,
-                    sync_result_builder.joined_room_ids,
-                    from_token=since_token,
-                )
+        users_that_have_changed = (
+            await self._device_handler.get_device_changes_in_shared_rooms(
+                user_id,
+                sync_result_builder.joined_room_ids,
+                from_token=since_token,
+                now_token=sync_result_builder.now_token,
             )
+        )
 
         # Step 1b, check for newly joined rooms
         for room_id in newly_joined_rooms: