defer wg.Done()

	req, err := http.NewRequest("GET", url, nil)
	if err != nil {
		return err
	}
	res, err := http.DefaultTransport.RoundTrip(req)
	if err != nil {
		return err
	}
	// Handle redirects.
	if res.StatusCode/100 == 3 {
		newURL, err := res.Location()
		if err != nil {
			return fmt.Errorf("resolving redirect: %v", err)
		}
		if !strings.HasPrefix(newURL.String(), *root) {