code: func ensureShared(path string) error {
  	sourceMount, optionalOpts, err := getSourceMount(path)
  	if err != nil {
  		return err
  	}
  	// Make sure source mount point is shared.
  	if !hasMountinfoOption(optionalOpts, sharedPropagationOption) {
  		return errors.Errorf("path %s is mounted on %s but it is not a shared mount", path, sourceMount)
  	}
  	return nil
  }
docs: // Ensure mount point on which path is mounted, is shared.