code: func (d *deployerImpl) setCurrentInstanceID(ctx context.Context, packageDir, instanceID string) error {
  	if err := common.ValidateInstanceID(instanceID, common.AnyHash); err != nil {
  		return err
  	}
  	if runtime.GOOS == "windows" {
  		return fs.EnsureFile(
  			ctx, d.fs, filepath.Join(packageDir, currentTxt),
  			strings.NewReader(instanceID))
  	}
  	return d.fs.EnsureSymlink(ctx, filepath.Join(packageDir, currentSymlink), instanceID)
  }
docs: // setCurrentInstanceID changes a pointer to currently installed instance ID.
  //
  // It takes a path to a package directory (.cipd/pkgs/<name>) as input.