Revert "Improve perf of sync device lists" (#17207)

Reverts element-hq/synapse#17191

1 files changed, 31 insertions, 6 deletions

diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 659499af75..2bd1b8de88 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1854,13 +1854,38 @@ class SyncHandler:
 
         # Step 1a, check for changes in devices of users we share a room
         # with
-        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,
-            )
+        #
+        # 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_rooms 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,
+                )
+            )
 
         # Step 1b, check for newly joined rooms
         for room_id in newly_joined_rooms: