public int available() {
        Entry entry;
        synchronized (lock) {
            if (queue == null) {
                return 0;
            }
            entry = queue.peek();
        }
        if (entry == null || entry.buf == null) {
            return 0;
        }
        return entry.buf.remaining();
    }
<p>Returns a lower-bound estimate on the number of bytes available to be {@link #read()} without blocking. If
the returned number is larger than zero, the next call to {@link #read()} is guaranteed to not block.</p>

@return The number of bytes available to be read without blocking.
jdisc_core/src/main/java/com/yahoo/jdisc/handler/ReadableContentChannel.java