code: public LocatedBlock getAdditionalBlock(String src,
                                                       String clientName,
                                                       List<Node> excludedNodes)
    throws IOException {
      return getAdditionalBlock(src, clientName, excludedNodes, null,
          -1, null, BlockMetaInfoType.NONE);
    }
docs: The client would like to obtain an additional block for the indicated
  filename (which is being written-to).  Return an array that consists
  of the block, plus a set of machines.  The first on this list should
  be where the client writes data.  Subsequent items in the list must
  be provided in the connection to the first datanode.
  <p/>
  Make sure the previous blocks have been reported by datanodes and
  are replicated.  Will return an empty 2-elt array if we want the
  client to "try again later".