code: func (r *Resolver) verifyPin(ctx context.Context, pin common.Pin) error {
  	promise := r.verifying.Get(ctx, pin, func(ctx context.Context) (interface{}, error) {
  		logging.Debugf(ctx, "Validating pin %s...", pin)
  		_, err := r.Client.DescribeInstance(ctx, pin, nil)
  		if err == nil {
  			logging.Debugf(ctx, "Pin %s successfully validated", pin)
  		} else {
  			logging.Debugf(ctx, "Failed to resolve instance info for %s: %s", pin, err)
  		}
  		return nil, err
  	})
  	_, err := promise.Get(ctx)
  	return err
  }
docs: // verifyPin returns nil if the given pin exists on the backend.