code: function(req, res) {
      var params = URL.parse(req.url, true).query;
      var oauth2_server_id = params.provider || this.config.default_server;
      var next_url = this.nexturl_query(req, params);
      this.redirects_for_login(oauth2_server_id, res, next_url);
    }
docs: Triggers redirects_for_login with next param if present in url query.