Function handshake

packages/engine.io/lib/server.ts:435–528

* Handshakes a new client. * * @param {String} transportName * @param {Object} req - the request object * @param {Function} closeConnection * * @protected

(
    transportName: TransportName,
    req: EngineRequest,
    closeConnection: ErrorCallback,
  )

   * @protected
   */
  protected async handshake(
    transportName: TransportName,
    req: EngineRequest,
    closeConnection: ErrorCallback,
  ) {
    const protocol = req._query.EIO === "4" ? 4 : 3; // 3rd revision by default
    if (protocol === 3 && !this.opts.allowEIO3) {
      debug("unsupported protocol version");
      this.emit("connection_error", {
        req,
        code: Server.errors.UNSUPPORTED_PROTOCOL_VERSION,
        message:
          Server.errorMessages[Server.errors.UNSUPPORTED_PROTOCOL_VERSION],
        context: {
          protocol,
        },
      });
      closeConnection(Server.errors.UNSUPPORTED_PROTOCOL_VERSION);
      return;
    }

    let id;
    try {
      id = await this.generateId(req);
    } catch (e) {
      debug("error while generating an id");
      this.emit("connection_error", {
        req,
        code: Server.errors.BAD_REQUEST,
        message: Server.errorMessages[Server.errors.BAD_REQUEST],
        context: {
          name: "ID_GENERATION_ERROR",
          error: e,
        },
      });
      closeConnection(Server.errors.BAD_REQUEST);
      return;
    }

    debug('handshaking client "%s"', id);

    try {
      var transport = this.createTransport(transportName, req);
      if ("polling" === transportName) {
        transport.maxHttpBufferSize = this.opts.maxHttpBufferSize;
        transport.httpCompression = this.opts.httpCompression;
      } else if ("websocket" === transportName) {
        transport.perMessageDeflate = this.opts.perMessageDeflate;
      }
    } catch (e) {
      debug('error handshaking to transport "%s"', transportName);
      this.emit("connection_error", {
        req,
        code: Server.errors.BAD_REQUEST,
        message: Server.errorMessages[Server.errors.BAD_REQUEST],
        context: {
          name: "TRANSPORT_HANDSHAKE_ERROR",
          error: e,