Function take 

pub fn take(
    folder: &Path,
    key: &str,
    seal: &dyn Sealer,
    now_ms: i64,
) -> Result<Taken, String>

One taken by the listener, once, under the folder’s lock: “taken” written and flushed before anything is sent, so that a crash after it leaves the doubt, never a second sending. Not due, or done already: nothing. Handed over more than TAKE_WITHIN_MS ago: “late”, not sent.