fix: store handshake headers on the socket object

Related: socketio/socket.io#5394

We could also have used the "connection" event:

```js
- engine.on("connection", (socket, { remoteAddress, headers }) => {});
+ engine.on("connection", (socket, request, server) => {});
```

In order to abstract the way each runtime gives access to the headers and the remote address of the client:

- Node.js: `IncomingMessage` object with `remoteAddress`
- Deno: `Request` native object and `Deno.ServeHandlerInfo`
- Bun: `Request` native object and `server.requestIP()`

That would be a breaking change though.

Author: darrachequesne

lib/server.ts

@@ -347,7 +347,7 @@ export class Server extends EventEmitter<
 
     debug(`new socket ${id}`);
 
-    const socket = new Socket(id, this.opts, transport);
+    const socket = new Socket(id, this.opts, transport, req);
     this.clients.set(id, socket);
 
     socket.once("close", (reason) => {

lib/socket.ts

@@ -40,6 +40,7 @@ export class Socket extends EventEmitter<
   public readonly id: string;
   public readyState: ReadyState = "opening";
   public transport: Transport;
+  public readonly request;
 
   private readonly opts: ServerOptions;
   private upgradeState: UpgradeState = "not_upgraded";
@@ -50,14 +51,27 @@ export class Socket extends EventEmitter<
   private pingIntervalTimer?: Timer;
   private pingTimeoutTimer?: Timer;
 
-  constructor(id: string, opts: ServerOptions, transport: Transport) {
+  constructor(
+    id: string,
+    opts: ServerOptions,
+    transport: Transport,
+    req: Request,
+  ) {
     super();
 
     this.id = id;
     this.opts = opts;
 
     this.transport = transport;
     this.bindTransport(transport);
+
+    // we store the headers of the handshake request, so that they are available in the `socket.handshake` attribute in
+    // the `socket.io` library
+    this.request = {
+      headers: Object.fromEntries(req.headers.entries()),
+      connection: {},
+    };
+
     this.onOpen();
   }
 

test/socket.test.ts

@@ -25,42 +25,46 @@ async function initSocketIOConnection() {
   return socket;
 }
 
-// imported from https://github.com/socketio/socket.io/tree/main/docs/socket.io-protocol/v5-test-suite
-describe("Socket.IO protocol", () => {
-  beforeAll(() => {
-    const io = new Server();
+function setup() {
+  const io = new Server();
 
-    const engine = new Engine({
-      path: "/socket.io/",
-      pingInterval: PING_INTERVAL,
-      pingTimeout: PING_TIMEOUT,
-    });
+  const engine = new Engine({
+    path: "/socket.io/",
+    pingInterval: PING_INTERVAL,
+    pingTimeout: PING_TIMEOUT,
+  });
 
-    io.bind(engine);
+  io.bind(engine);
 
-    io.on("connection", (socket) => {
-      socket.emit("auth", socket.handshake.auth);
+  io.on("connection", (socket) => {
+    expect(socket.handshake.headers).toContainKey("host");
 
-      socket.on("message", (...args) => {
-        socket.emit.apply(socket, ["message-back", ...args]);
-      });
+    socket.emit("auth", socket.handshake.auth);
 
-      socket.on("message-with-ack", (...args) => {
-        const ack = args.pop();
-        ack(...args);
-      });
+    socket.on("message", (...args) => {
+      socket.emit.apply(socket, ["message-back", ...args]);
     });
 
-    io.of("/custom").on("connection", (socket) => {
-      socket.emit("auth", socket.handshake.auth);
+    socket.on("message-with-ack", (...args) => {
+      const ack = args.pop();
+      ack(...args);
     });
+  });
 
-    Bun.serve({
-      port: 3001,
-      ...engine.handler(),
-    });
+  io.of("/custom").on("connection", (socket) => {
+    socket.emit("auth", socket.handshake.auth);
   });
 
+  Bun.serve({
+    port: 3001,
+    ...engine.handler(),
+  });
+}
+
+// imported from https://github.com/socketio/socket.io/tree/main/docs/socket.io-protocol/v5-test-suite
+describe("Socket.IO protocol", () => {
+  beforeAll(() => setup());
+
   describe("connect", () => {
     it("should allow connection to the main namespace", async () => {
       const socket = createWebSocket(