Function CloneFile

tools/util_windows.go:85–129

(writer io.Writer, reader io.Reader)

}

func CloneFile(writer io.Writer, reader io.Reader) (success bool, err error) {
	dst, dstIsFile := writer.(*os.File)
	src, srcIsFile := reader.(*os.File)
	if !(dstIsFile && srcIsFile) {
		return false, nil
	}

	srcStat, err := src.Stat()
	if err != nil {
		return
	}

	fileSize := srcStat.Size()

	err = dst.Truncate(fileSize) // set file size. There is a requirement "The destination region must not extend past the end of file."
	if err != nil {
		return
	}

	offset := int64(0)

	// Requirement
	// * The source and destination regions must begin and end at a cluster boundary. (4KiB or 64KiB)
	// * cloneRegionSize less than 4GiB.
	// see https://docs.microsoft.com/windows/win32/fileio/block-cloning

	// Clone first xGiB region.
	for ; offset+GiB < fileSize; offset += GiB {
		err = callDuplicateExtentsToFile(dst, src, offset, GiB)
		if err != nil {
			return false, err
		}
	}

	// Clone tail. First try with 64KiB round up, then fallback to 4KiB.
	for _, cloneRegionSize := range availableClusterSize {
		err = callDuplicateExtentsToFile(dst, src, offset, roundUp(fileSize-offset, cloneRegionSize))
		if err != nil {
			continue
		}
		break
	}

	return err == nil, err
}

// call FSCTL_DUPLICATE_EXTENTS_TO_FILE IOCTL
// see https://docs.microsoft.com/en-us/windows/win32/api/winioctl/ni-winioctl-fsctl_duplicate_extents_to_file