public void write(ByteBuffer buf) {
        numPendingCompletions.incrementAndGet();
        try {
            out.write(buf, completionHandler);
        } catch (Throwable t) {
            completeExceptionally(t);
            throw t;
        }
    }
<p>Writes to the underlying {@link ContentChannel}. If {@link CompletionHandler#failed(Throwable)} is called,
either of the get() methods will rethrow that Throwable.</p>

@param buf The ByteBuffer to write.
jdisc_core/src/main/java/com/yahoo/jdisc/handler/FastContentWriter.java