download cert chain

std::vector<std::string> fetchCertChain(const std::string& host, int port) {
      std::vector<std::string> certPaths;

      SSL_library_init();
      SSL_load_error_strings();

      SSL_CTX* ctx = nullptr;
      SSL* ssl = nullptr;
      BIO* bio = nullptr;

      auto cleanup = [&]() {
          if (bio)
            BIO_free_all(bio);

          if (ctx)
            SSL_CTX_free(ctx);
      };

      const SSL_METHOD* method = TLS_client_method();
      ctx = SSL_CTX_new(method);

      bio = BIO_new_ssl_connect(ctx);
      BIO_get_ssl(bio, &ssl);
      SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);

      std::string target = host + ":" + std::to_string(port);
      const auto pos = target.find("://");
      if (pos != std::string::npos) {
        target.erase(0, pos + 3);
      }

      BIO_set_conn_hostname(bio, target.c_str());

      if (BIO_do_connect(bio) <= 0) {
        std::cerr << "Connection failed\n";
        cleanup();
        return certPaths;
      }

      STACK_OF(X509)* certs = SSL_get_peer_cert_chain(ssl);
      if (!certs) {
        std::cerr << "No cert chain received.\n";
        cleanup();
        return certPaths;
      }

      for (int i = 0; i < sk_X509_num(certs); ++i) {
        X509* cert = sk_X509_value(certs, i);
//        X509_NAME* subj = X509_get_subject_name(cert);
        const std::string name = getCertSha1(cert);
//        std::string name = getSubjectField(cert, NID_commonName);
//        if (name.empty())
//          name = getCertSha1(cert);
//        std::cout << "name: " << name << "\n";
//        const std::string name = "cert_" + std::to_string(i) + ".pem";
////        const std::string name = "cert_" + std::to_string(i) + ".pem";
        const fs::path certPath = getPath() / name;
//        fs::path derPath{certPath};
//        derPath.replace_extension(".der");
        FILE* fp = fopen(certPath.c_str(), "w");
        if (fp) {
          PEM_write_X509(fp, cert);
          fclose(fp);
          certPaths.push_back(certPath);
//          ::convertPemToDer(pemPath, derPath);
          std::cout << "Saved: " << certPath << "\n";
        }
      }

//      BIO_free_all(bio);
//      SSL_CTX_free(ctx);
      cleanup();

      return certPaths;
    }