No messages sent to guests by the external signaling server when rooms are updated

If I am not mistaken this needs to be fixed both in Talk and in the external signaling server; in Talk the update messages should include the session IDs of the guests and in the external signaling server those IDs should be used to send the same messages currently sent to users.

Another option could be that the update messages include the full list of properties besides the name and type; update messages trigger Room.UpdateProperties in the external signaling server, which would send room messages to the clients when the known properties are updated (but this would require to change the external signaling server code whenever a new updateable property is added). This is why currently changing the room name sends a room message to guests (which triggers a synchronization that updates the UI) even if BackendNotifier::roomModified does not provide the guest sessions, but no messages are sent when other properties change.

@fancycode

How to test

• Setup the external signaling server
• Create a public conversation as a user
• Copy the conversation link
• In a private window, open the copied link
• Back to the user, enable the conversation for moderators only
• Change again to the private window

Expected result

The lobby is immediately shown in the private window

Actual result

The lobby is not immediately shown in the private window; eventually it will be shown, but only due to the periodic forced synchronization. You can comment that line to see the lack of updates more clearly.

Reproduce the public-conversation flow with the external signaling server, then inspect BackendNotifier::roomModified and the signaling.js room synchronization paths linked in the issue. Trace how update messages reach Room.UpdateProperties and guests, and verify that changing moderator-only access immediately shows the lobby in the private window without relying on periodic forced synchronization.