Function drain_in 

pub fn drain_in(
    dir: &Path,
    send: impl FnMut(&Queued) -> Sent,
) -> Result<Drained>

Send each waiting note in dir through send, oldest first. A kept or refused note leaves the queue; the first one that has to wait stops the drain, and it and every later note stay, in order.

Errors

The queue file cannot be rewritten.