Function download

pub(crate) async fn download<R: Runtime>(
    dirmgr: Weak<DirMgr<R>>,
    state: &mut Box<dyn DirState>,
    schedule: &mut TaskSchedule<R>,
    attempt_id: AttemptId,
    on_usable: &mut Option<Sender<()>>,
) -> Result<()>

Download information into a DirState state machine until it is “complete”, or until we hit a non-recoverable error.

Use dirmgr to load from the cache or to launch downloads.

Keep resetting the state as needed.

The first time that the state becomes “usable”, notify the sender in on_usable.