@Override
        public void completed() {
            numPendingCompletions.decrementAndGet();
            if (closed.get() && numPendingCompletions.get() == 0) {
                complete(true);
            }
        }
<p>Closes the underlying {@link ContentChannel}. If {@link CompletionHandler#failed(Throwable)} is called,
either of the get() methods will rethrow that Throwable.</p>
jdisc_core/src/main/java/com/yahoo/jdisc/handler/FastContentWriter.java