Events before disconnection dropped

[asideofcode-dev]

Describe the bug
In a Socket.IO application, events emitted just before a client disconnects are not properly handled by specific on(...) listeners. While the onAny(...) listener detects these events, the specific on(...) listeners do not. This issue occurs with any event emitted just before disconnection. The reason onAny(...) works is that it runs before the dispatch, which relies on process.nextTick.

To Reproduce

Socket.IO server version: 4.x.x (replace with your version)

Server

import { Server } from "socket.io";

const io = new Server(3000, {});

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

  // Example event listener (applies to any event)
  socket.on("exampleEvent", () => {
    console.log(`exampleEvent received before disconnect ${socket.id}`);
  });

  // Using onAny listener
  socket.onAny((event, ...args) => {
    console.log(`onAny received ${event} before disconnect ${socket.id}`);
  });

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

Socket.IO client version: 4.x.x (replace with your version)

Client

import { io } from "socket.io-client";

const socket = io("ws://localhost:3000/", {});

socket.on("connect", () => {
  console.log(`connect ${socket.id}`);
  
  // Emit any event just before disconnecting
  socket.emit("exampleEvent");
  socket.disconnect();
});

socket.on("disconnect", () => {
  console.log("disconnect");
});

Expected behavior
The on listener for any event (e.g., exampleEvent) should detect and handle the event before disconnection. Currently, the onAny listener detects the event, but the specific on(...) listener does not. This occurs because onAny runs before the dispatch, which relies on process.nextTick, allowing it to detect the event before disconnection, see

socket.io/packages/socket.io/lib/socket.ts

Lines 559 to 575 in 175a2c5

	private onevent(packet: Packet): void {
	const args = packet.data || [];
	debug("emitting event %j", args);
	if (null != packet.id) {
	debug("attaching ack callback to event");
	args.push(this.ack(packet.id));
	}
	if (this._anyListeners && this._anyListeners.length) {
	const listeners = this._anyListeners.slice();
	for (const listener of listeners) {
	listener.apply(this, args);
	}
	}
	this.dispatch(args);
	}

.

Platform:

Not relevant

Additional context
This issue affects any event emitted just before disconnection. The onAny listener works because it operates before the event dispatch process, which uses process.nextTick. As a result, events are detected by onAny but missed by the specific on(...) listeners, which rely on the dispatch mechanism that happens after disconnection begins.

[darrachequesne]

Hi!

Unfortunately, I wasn't able to reproduce the issue: https://github.com/socketio/socket.io-fiddle/tree/issues/socket.io/5213

Either with [email protected] or [email protected].

The socket._onpacket() method is also within a process.nextTick() here, which should prevent the issue. Isn't that the case for you?

[asideofcode-dev]

@darrachequesne Thanks for setting up the fiddle. I just cloned it and visited the index.html page. The logs on the server are as follows:

server listening at http://localhost:1234
connect Pvc2MyvdRD_Z4K0cAAAB
onAny event
disconnect Pvc2MyvdRD_Z4K0cAAAB due to client namespace disconnect

The onevent line is missing from https://github.com/socketio/socket.io-fiddle/blob/issues/socket.io/5213/server.js#L33. Is it present in your tests ?

[darrachequesne]

Yes it is:

server listening at http://localhost:3000
connect NQN6DbbcOZBzIYFTAAAB
onAny event
onevent
disconnect NQN6DbbcOZBzIYFTAAAB due to client namespace disconnect
connect 8ZYrenG0SM1w85vMAAAD
onAny event
onevent
disconnect 8ZYrenG0SM1w85vMAAAD due to client namespace disconnect

Ubuntu 22.04, Node.js v22.8.0 and v20.15.1.

[asideofcode-dev]

OSX, Node v20.16.0 and Debian Bookworm, Node v22.6.0 both show this issue for me. How very strange!

[asideofcode-dev]

Maybe easier to reproduce in a container

➜  socket.io-fiddle git:(issues/socket.io/5213) ✗ docker run -p 1234:1234 --rm -it -v $PWD:$PWD -w $PWD node:23-bookworm bash
root@952ad7e0d8bb:/Users/m1/Desktop/repos/socket.io-fiddle# PORT=1234 node ./server.js 
server listening at http://localhost:1234
connect KgT7cuONDu7NGHRoAAAB
onAny event
disconnect KgT7cuONDu7NGHRoAAAB due to client namespace disconnect

[asideofcode-dev]

Ping @darrachequesne , thoughts on this ?