第六节 WriteTo()和CopyFile()实现

  1. // WriteTo writes the entire database to a writer.
  2. // If err == nil then exactly tx.Size() bytes will be written into the writer.
  3. func (tx *Tx) WriteTo(w io.Writer) (n int64, err error) {
  4.     // Attempt to open reader with WriteFlag
  5.     f, err := os.OpenFile(tx.db.path, os.O_RDONLY|tx.WriteFlag, 0)
  6.     if err != nil {
  7.         return 0, err
  8.     }
  9.     defer func() { _ = f.Close() }()
  11.     // Generate a meta page. We use the same page data for both meta pages.
  12.     buf := make([]byte, tx.db.pageSize)
  13.     page := (*page)(unsafe.Pointer(&buf[0]))
  14.     page.flags = metaPageFlag
  15.     *page.meta() = *tx.meta
  17.     // Write meta 0.
  18.     page.id = 0
  19.     page.meta().checksum = page.meta().sum64()
  20.     nn, err := w.Write(buf)
  21.     n += int64(nn)
  22.     if err != nil {
  23.         return n, fmt.Errorf("meta 0 copy: %s", err)
  24.     }
  26.     // Write meta 1 with a lower transaction id.
  27.     page.id = 1
  28.     page.meta().txid -= 1
  29.     page.meta().checksum = page.meta().sum64()
  30.     nn, err = w.Write(buf)
  31.     n += int64(nn)
  32.     if err != nil {
  33.         return n, fmt.Errorf("meta 1 copy: %s", err)
  34.     }
  36.     // Move past the meta pages in the file.
  37.     if _, err := f.Seek(int64(tx.db.pageSize*2), os.SEEK_SET); err != nil {
  38.         return n, fmt.Errorf("seek: %s", err)
  39.     }
  41.     // Copy data pages.
  42.     wn, err := io.CopyN(w, f, tx.Size()-int64(tx.db.pageSize*2))
  43.     n += wn
  44.     if err != nil {
  45.         return n, err
  46.     }
  48.     return n, f.Close()
  49. }
  51. // CopyFile copies the entire database to file at the given path.
  52. // A reader transaction is maintained during the copy so it is safe to continue
  53. // using the database while a copy is in progress.
  54. func (tx *Tx) CopyFile(path string, mode os.FileMode) error {
  55.     f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, mode)
  56.     if err != nil {
  57.         return err
  58.     }
  60.     err = tx.Copy(f)
  61.     if err != nil {
  62.         _ = f.Close()
  63.         return err
  64.     }
  65.     return f.Close()
  66. }