Function commit_now 

fn commit_now(
    payload: &Payload,
    engine: &mut Engine<'_>,
    report: &Reporter<'_>,
) -> (Plan, bool)

Write now, or take an ordering slot behind what is already waiting.

A unit that is due now on an empty queue is not pushed at all — it is written inline in the read arm, which is today’s code and today’s cost. On a non-empty queue it must be pushed, or it overtakes the units ahead of it; the deque is what holds it back, and its own deadline stays now (see PendingQueue::push) so that it goes out the instant the units ahead of it have.