Function try_spawn_blocking 

pub fn try_spawn_blocking<F, T>(
    function: F,
) -> Result<JoinHandle<T>, (RuntimeConfigError, F)>
where
    F: FnOnce() -> T + Send + 'static,
    T: Send + 'static,

Submit blocking work without consuming it when the runtime is not accepting work.

Lifecycle owners can retain the returned closure and retry it after start completes a runtime restart.