* than would be returned by reading all of the bytes (for example, some special files may return
   * a size of 0 despite actually having content when read).
   *
   * <p>In either case, for mutable sources such as files, a subsequent read may return a different
   * number of bytes if the contents are changed.
   *
   * @throws IOException if an I/O error occurs while reading the size of this source
   */