vlc_mutex_lock( &p_em->object_lock );
ARRAY_APPEND( p_em->listeners_groups, listeners_group );
vlc_mutex_unlock( &p_em->object_lock );
vlc_mutex_lock( &p_em->object_lock );
ARRAY_APPEND( p_em->listeners_groups, listeners_group );
vlc_mutex_unlock( &p_em->object_lock );