Function drain_honouring_release_times 

pub(crate) async fn drain_honouring_release_times<S, F>(
    pending: &mut PendingQueue,
    send: &mut S,
    cancel: &CancellationToken,
    on_shape: F,
) -> Result<DrainOutcome, EgressError>
where
    S: EgressSink,
    F: FnMut(ShapeReport),

Write everything queued at its release time, racing cancellation.

The shape both release-honouring drains take — the FIN drain in the read arm’s None branch and the one a terminal runs before it resets. Both sit inside select! arm bodies, which are not preemptible, so writing either as a plain while let loop would let a Hold on a gate nobody releases pin session teardown for up to EgressConfig::max_hold — precisely the failure the deque was chosen over a writer task to avoid.

The inner select! is biased so cancellation wins deterministically when both are ready. Unbiased, a cancelled session could keep picking the release branch, find nothing due, and spin.

Release lateness is not sampled here: this drain honours release times only until cancellation, and recording a teardown flush as a timing sample would report teardown as a timing failure.

Shaping is reported here

on_shape is called once per ShapeReport the drain’s own PendingQueue::pop_next_due produces, and it exists because this loop is a full release seam and not a teardown flush: on a shaped queue every pop below debits a token bucket and every clamp or expiry is decided here. The FIN path — header, a few objects, FIN, the ordinary MoQT subgroup shape — reaches the wire through this function and not through release_due_units, so a drain that swallowed its reports would apply the whole profile to the normal case and say nothing about it. That is the silent no-op this signature exists to prevent, and the reason the parameter is not optional: a caller cannot forget what it has to name.

Passing a no-op closure is correct only where no report can exist — every #[cfg(test)] caller in this module builds an unshaped queue, and an unshaped queue’s pop_next_due never reaches a scheduler.