docs: def validate_redirect_url(next_url):
          """
          Returns the next_url path if next_url matches allowed hosts.
          """
          if not next_url:
              return None
          parts = urlparse(next_url)
          if parts.netloc:
              domain, _ = split_domain_port(parts.netloc)
              allowed_hosts = (['*'] if django_settings.DEBUG
                  else django_settings.ALLOWED_HOSTS)
              if not (domain and validate_host(domain, allowed_hosts)):
                  return None
          return urlunparse(("", "", parts.path,
              parts.params, parts.query, parts.fragment))