1 files changed, 52 insertions, 17 deletions
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index b79c551703..c05a170c55 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -123,23 +123,28 @@ class DeviceWorkerHandler:
 
         return device
 
-    @trace
-    @measure_func("device.get_user_ids_changed")
-    async def get_user_ids_changed(
-        self, user_id: str, from_token: StreamToken
-    ) -> JsonDict:
-        """Get list of users that have had the devices updated, or have newly
-        joined a room, that `user_id` may be interested in.
+    async def get_device_changes_in_shared_rooms(
+        self, user_id: str, room_ids: Collection[str], from_token: StreamToken
+    ) -> Collection[str]:
+        """Get the set of users whose devices have changed who share a room with
+        the given user.
         """
+        changed_users = await self.store.get_device_list_changes_in_rooms(
+            room_ids, from_token.device_list_key
+        )
 
-        set_tag("user_id", user_id)
-        set_tag("from_token", from_token)
-        now_room_key = self.store.get_room_max_token()
+        if changed_users is not None:
+            # We also check if the given user has changed their device. If
+            # they're in no rooms then the above query won't include them.
+            changed = await self.store.get_users_whose_devices_changed(
+                from_token.device_list_key, [user_id]
+            )
+            changed_users.update(changed)
+            return changed_users
 
-        room_ids = await self.store.get_rooms_for_user(user_id)
+        # If the DB returned None then the `from_token` is too old, so we fall
+        # back on looking for device updates for all users.
 
-        # First we check if any devices have changed for users that we share
-        # rooms with.
         users_who_share_room = await self.store.get_users_who_share_room_with_user(
             user_id
         )
@@ -153,6 +158,27 @@ class DeviceWorkerHandler:
             from_token.device_list_key, tracked_users
         )
 
+        return changed
+
+    @trace
+    @measure_func("device.get_user_ids_changed")
+    async def get_user_ids_changed(
+        self, user_id: str, from_token: StreamToken
+    ) -> JsonDict:
+        """Get list of users that have had the devices updated, or have newly
+        joined a room, that `user_id` may be interested in.
+        """
+
+        set_tag("user_id", user_id)
+        set_tag("from_token", from_token)
+        now_room_key = self.store.get_room_max_token()
+
+        room_ids = await self.store.get_rooms_for_user(user_id)
+
+        changed = await self.get_device_changes_in_shared_rooms(
+            user_id, room_ids, from_token
+        )
+
         # Then work out if any users have since joined
         rooms_changed = self.store.get_rooms_that_changed(room_ids, from_token.room_key)
 
@@ -237,10 +263,19 @@ class DeviceWorkerHandler:
                         break
 
         if possibly_changed or possibly_left:
-            # Take the intersection of the users whose devices may have changed
-            # and those that actually still share a room with the user
-            possibly_joined = possibly_changed & users_who_share_room
-            possibly_left = (possibly_changed | possibly_left) - users_who_share_room
+            possibly_joined = possibly_changed
+            possibly_left = possibly_changed | possibly_left
+
+            # Double check if we still share rooms with the given user.
+            users_rooms = await self.store.get_rooms_for_users_with_stream_ordering(
+                possibly_left
+            )
+            for changed_user_id, entries in users_rooms.items():
+                if any(e.room_id in room_ids for e in entries):
+                    possibly_left.discard(changed_user_id)
+                else:
+                    possibly_joined.discard(changed_user_id)
+
         else:
             possibly_joined = set()
             possibly_left = set()