code: func (r *runner) setupWorkDir(workDir string) error {
  	switch _, err := os.Stat(workDir); {
  	case err == nil:
  		return errors.Reason("workdir %q already exists; it must not", workDir).Err()
  
  	case os.IsNotExist(err):
  		// good
  
  	default:
  		return err
  
  	}
  
  	return errors.Annotate(os.MkdirAll(workDir, 0700), "failed to create %q", workDir).Err()
  }
docs: // setupWorkDir creates a work dir.
  // If workdir already exists, returns an error.