Publish not being received by client when unsubscribing [from all topics] in the same tick

myndzi commented 1 year ago

Pub/sub is now a lot more predictable and always guarantees strict ordering inside topics, across topics and between WebSocket::send and WebSocket::publish / App::publish. Subscription and unsubscription is always guaranteed to be strictly ordered with respect to publish. Support for MQTT wildcards has been removed.

I have a case where I'm working on the backend for an existing application; I'm using pub/sub to represent game lobbies. When the host leaves the room, a control message is sent to the topic, and then each subscribed client is unsubscribed. However, the control message is not being sent.

I managed to thin it down to a minimum reproducible version, though my initial attempts seemed to work as expected. I was unable to determine the significant difference between my initial attempts and the repro case below. I've been at it some hours, hopefully this is actually a repro case and not user error.

const uWS = require('uWebSockets.js');
const { WebSocket } = require('ws');

const app = uWS
  .App()
  .ws('/', {
    open: (ws) => {
      ws.subscribe('test');
      app.publish('test', 'hi from pubsub');
      // swap comments on the next two lines - will change between success and failure
      ws.unsubscribe('test');
      // setImmediate(() => ws.unsubscribe('test'));
      ws.send('???');
    }
  })
  .listen(3300, () => {
    const wsClient = new WebSocket('ws://localhost:3300/');
    wsClient.addEventListener('message', (evt) => {
      const msg = Buffer.from(evt.data).toString();
      console.log('client received from server:', msg);
      if (msg === '???') app.close();
    });
  });

$ npm ls
uws-bug@1.0.0 /home/myndzi/uws-bug
├── uWebSockets.js@20.32.0 (git+ssh://git@github.com/uNetworking/uWebSockets.js.git#21c0e41a662c27bcc29b64f3e9d7f0bdaa6d7ee7)
└── ws@8.14.2

$ node --version
v20.8.1

myndzi commented 1 year ago

Note: if the client is subscribed to a different pubsub topic, it receives the result it was intended to. This produces the expected result:

const uWS = require('uWebSockets.js');
const { WebSocket } = require('ws');

const app = uWS
  .App()
  .ws('/', {
    open: (ws) => {
      ws.subscribe('other'); // new
      ws.subscribe('test');
      app.publish('test', 'hi from pubsub');
      ws.unsubscribe('test');
      ws.send('???');
    }
  })
  .listen(3300, () => {
    const wsClient = new WebSocket('ws://localhost:3300/');
    wsClient.addEventListener('message', (evt) => {
      const msg = Buffer.from(evt.data).toString();
      console.log('client received from server:', msg);
      if (msg === '???') app.close();
    });
  });

Edit: come to think of it, this was probably the difference-maker in my attempted reproduction, since I had a bug in my actual code where I forgot to subscribe users to the lobby, so I wasn't getting the control message since the users, once unsubscribed, were present in 0 pubsub topics. But since I had intended them to be in the lobby, my repro code didn't have that bug and worked unexpectedly...