+ /** Called whenever a user's write buffer has been completely sent.
+ * This is called when the user's write buffer is completely empty, and
+ * there are no more pending bytes to be written and no pending write events
+ * in the socket engine's queue. This may be used to refill the buffer with
+ * data which is being spooled in a controlled manner, e.g. LIST lines.
+ * @param user The user who's buffer is now empty.
+ */