Build a set of who we are interested in first and foremost

synapse/handlers/e2e_keys.py

@@ -300,17 +300,17 @@ class E2eKeysHandler:
         # queries. We use the more efficient batched query_client_keys for all
         # remaining users
         user_ids_updated = []
-        for (user_id, device_list) in destination_query.items():
-            if user_id in user_ids_updated:
-                continue
 
-            if device_list:
-                continue
-
-            room_ids = await self.store.get_rooms_for_user(user_id)
-            if not room_ids:
-                continue
+        # Perform a user device resync for each user only once and only as long as:
+        # - they have an empty device_list
+        # - they are in some rooms that this server can see
+        users_to_resync_devices = {
+            user_id
+            for (user_id, device_list) in destination_query.items()
+            if (not device_list) and (await self.store.get_rooms_for_user(user_id))
+        }
 
+        for user_id in users_to_resync_devices:
             # We've decided we're sharing a room with this user and should
             # probably be tracking their device lists. However, we haven't
             # done an initial sync on the device list so we do it now.