Client emits initial event twice inside 'connect' handler when retries are enabled.

[backendsuraj]

Describe the bug

When the retries option is enabled in the Socket.IO client, the client emits the initial event twice inside the 'connect' handler. However, this issue does not occur when the retries option is not provided.

To Reproduce

Please fill the following code example:

Socket.IO server version: 4.8.1

Server

const io = new Server(httpServer, {});

io.on("connection", (socket) => {
  console.log(`connect ${socket.id}`);

  socket.on('data', (data, callback)=> {
    console.log('Incoming message from client: ', data);
    callback({
      success: true,
      message: 'Acknowledged...'
    });
  });

  socket.on("disconnect", (reason) => {
    console.log(`disconnect ${socket.id} due to ${reason}`);
  });
});

Socket.IO client version: 4.8.1

Client

const socket = io(`http://localhost:${port}`, {
  retries: 2,
  ackTimeout: 3000,
  transports: ["websocket"],
});

socket.on("connect", () => {
  console.log(`connect ${socket.id}`);
  const msg1 = 'Hello!!!';
  const msg2 = 'Byeee';
  socket.emit('data', msg1);
  socket.emit('data', msg2);
  console.log('Emitted msg: ', msg1);
  console.log('Emitted msg: ', msg2);
});

Expected behavior
The event inside the 'connect' handler should be emitted only once, regardless of whether the retries option is enabled or disabled.

Platform:

• Device: Windows/google chrome
• OS: Windows/google chrome

Additional context
Console output:

I debugged this issue and it seems like:

After the client emits the connect event, the event queue is drained with the force option set to true. This action retriggers events that were already added to the queue & awaiting acknowledgement due to the retries option. As a result, the initial event is emitted twice.

[darrachequesne]

Hi! I think that's because the client expects a null as a first argument for the callback:

io.on("connection", (socket) => {
  // [...]
  socket.on('data', (data, callback)=> {
    console.log('Incoming message from client: ', data);
    callback(null, {
      success: true,
      message: 'Acknowledged...'
    });
  });
  // [...]
});

Source

socket.io/packages/socket.io-client/lib/socket.ts

Line 548 in 7427109

const hasError = err !== null;

It seems it's not properly documented though...