code: void insertIntoList(BlockInfo head, int headIndex, BlockInfo tail, int tailIndex, int count) {
      if (head == null)
        return;
      
      // connect tail to now-head
      tail.setNext(tailIndex, blockList);
      if (blockList != null)
        blockList.setPrevious(blockList.findDatanode(this), tail);
      
      // create new head
      blockList = head;
      blockList.setPrevious(headIndex, null);
      
      // add new blocks to the count
      numOfBlocks += count;
    }
docs: Adds blocks already connected into list, to this descriptor's blocks.
  The blocks in the input list already have this descriptor inserted to them.
  Used for parallel initial block reports.