Send device list updates out to servers in partially joined rooms (#13874)

Use the provided list of servers in the room from the `/send_join`
response, since we will not know which users are in the room.  This
isn't sufficient to ensure that all remote servers receive the right
device list updates, since the `/send_join` response may be inaccurate
or we may calculate the membership state of new users in the room
incorrectly.

Signed-off-by: Sean Quah <seanq@matrix.org>

1 files changed, 5 insertions, 1 deletions

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

index 901e2310b7..6566b3bf3d 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -688,11 +688,15 @@ class DeviceHandler(DeviceWorkerHandler):
                     # Ignore any users that aren't ours
                     if self.hs.is_mine_id(user_id):
                         hosts = set(
-                            await self._storage_controllers.state.get_current_hosts_in_room(
+                            await self._storage_controllers.state.get_current_hosts_in_room_or_partial_state_approximation(
                                 room_id
                             )
                         )
                         hosts.discard(self.server_name)
+                        # For rooms with partial state, `hosts` is merely an
+                        # approximation. When we transition to a full state room, we
+                        # will have to send out device list updates to any servers we
+                        # missed.
 
                     # Check if we've already sent this update to some hosts
                     if current_stream_id == stream_id: