What is srcF for in extractTi ?

[zhiqiangxu]

I don't see what os.Open and srcF.Close are for, are these really necessary?

func (bkd *BkdTree) extractTi(dstF *os.File, idx int) (err error) {
    if bkd.trees[idx].meta.NumPoints <= 0 {
        return
    }
    srcF, err := os.Open(bkd.TiPath(idx))
    if err != nil {
        err = errors.Wrap(err, "")
        return
    }
    defer srcF.Close()

    //depth-first extracting from the root node
    meta := &bkd.trees[idx].meta
    err = bkd.extractNode(dstF, bkd.trees[idx].data, meta, int(meta.RootOff))
    return
}