public void clean() {
            // the lock protects removal from a concurrent put which could otherwise mutate the
            // queue after it has been removed from the map
            synchronized (unsent) {
                unsent.values().removeIf(ConcurrentLinkedQueue::isEmpty);
            }
        }
Return whether the caller is still awaiting an IO event.
@return true if so, false otherwise.
clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerNetworkClient.java