Socket.IO with uWebSockets: "initial_headers" event not being emitted

[AKibMg]

Describe the bug
Seems like the "initial_headers" (and also "headers") event is never emitted when using Socket.IO with uWebSockets. Need a way to attach headers to the http handshake response.

To Reproduce
socket.io v4.8.1
uWebSockets.js v20.51.0

Server

server.mjs 

import { App } from "uWebSockets.js";
import { Server } from "socket.io";

const app = new App();
const io = new Server({
  serveClient: false,
  transports: ["websocket"],
});

io.attachApp(app);

io.engine.on("initial_headers", (headers, req) => {
  console.log("initial headers event emitted");
});

io.on("connection", (socket) => {
  console.log("connected");
});

app.listen(3000, (token) => {
  if (token) {
    console.log("Server listening on port 3000");
  } else {
    console.log("Failed to listen on port 3000");
  }
});

Client

const socket = io("http://localhost:3000", {
  transports: ["websocket"],
  reconnection: true,
});

Expected behavior
Expecting console to log "initial headers event emitted", but only getting "connected" and "Server listening on port 3000".

[ilkyy]

only 'polling' requests are emitted by initial_headers event, downgrading uwebsocket to v20.30.0 or socket.io to 4.5.0 does not worked.

[darrachequesne]

I could indeed reproduce the issue.

The test here is disabled for the engine based on uWebSockets:

socket.io/packages/engine.io/test/server.js

Lines 3600 to 3606 in 72d61da

	it("should emit a 'initial_headers' event (websocket)", function (done) {
	if (
	process.env.EIO_WS_ENGINE === "eiows" ||
	process.env.EIO_WS_ENGINE === "uws"
	) {
	return this.skip();
	}

For the default engine, we use the headers event emitted by the ws server:

socket.io/packages/engine.io/lib/server.ts

Lines 690 to 707 in 72d61da

	this.ws.on("headers", (headersArray, req) => {
	// note: 'ws' uses an array of headers, while Engine.IO uses an object (response.writeHead() accepts both formats)
	// we could also try to parse the array and then sync the values, but that will be error-prone
	const additionalHeaders = req[kResponseHeaders] || {};
	delete req[kResponseHeaders];
	const isInitialRequest = !req._query.sid;
	if (isInitialRequest) {
	this.emit("initial_headers", additionalHeaders, req);
	}
	this.emit("headers", additionalHeaders, req);
	debug("writing headers: %j", additionalHeaders);
	Object.keys(additionalHeaders).forEach((key) => {
	headersArray.push(`${key}: ${additionalHeaders[key]}`);
	});
	});