Function spawn_blocking_cancellable_flatten 

pub fn spawn_blocking_cancellable_flatten<F, T>(
    f: F,
) -> impl Future<Output = Result<T>>
where
    F: FnOnce(&Cancellable) -> Result<T> + Send + 'static,
    T: Send + 'static,

A wrapper around spawn_blocking_cancellable that flattens nested results.