function iterator.drain
Stop consuming: unsubscribe and drop whatever is buffered. Idempotent. Releases the producer’s blocking-send path for this subscriber, so a task that decides mid-build it no longer wants entries stops holding the end-of-build join open.
function iterator.try_pop
Non-blocking pop of the next entry this handle’s
kinds= filter wants, or None when nothing is buffered yet. Entries of other kinds are skipped without being seen from AXL.
