Function view_owning_source 

pub fn view_owning_source(
    views: &MultibufferRegistryHandle,
    source: BufferId,
) -> Option<Arc<MultibufferDocumentHandle>>

OA.23b: the view that owns source, across every registered view.

None when no view has it — an ordinary answer, because the caller is acting on an id a guest handed back and a view can close between the two.