code: func tar2ext4Reexec() {
  	size, err := tar2ext4Actual(os.Args[1], os.Stdin)
  	if err != nil {
  		fmt.Fprint(os.Stderr, err)
  		os.Exit(1)
  	}
  	fmt.Fprint(os.Stdout, size)
  }
docs: // tar2ext4Reexec is the re-exec entry point for writing a layer from a tar file