Function lease_next 

pub fn lease_next(
    queue: &mut Vec<TaskProgress>,
    in_flight: &InFlightSet,
    worker: &str,
) -> Option<TaskProgress>

Lease the next task of queue to worker: stamp the attempt’s dispatch time (the lease clock starts now — KNOWN_ISSUES D-27), record the worker as this attempt’s (D-26), and record it in flight. A re-queued task whose late result already finished it is dropped instead (it is not converted twice); the next one is tried. None once the queue is empty.