Function spawn_hotplug_watch 

pub fn spawn_hotplug_watch(
    rt: &Handle,
    commands: Sender<Command>,
) -> Result<(), Error>

Start watching USB and feed what it sees to the supervisor.

Devices already plugged in arrive as HotplugEvent::Arrived on the first poll, so this is the only enumeration the daemon does. Listing devices separately at startup would mount each of them twice.

Errors

Returns an error if the OS refuses to set up USB hotplug notifications, which leaves the daemon with nothing to do.