fix(eio): make the upgrade response an EventEmitter

WebSocketResponse exposes a subset of http.ServerResponse so that Express
middlewares can be applied to an upgrade request, but it was a plain
object. Any middleware that observes the response lifecycle — pino-http
and most other request loggers do, via res.on('close') — threw
"TypeError: res.on is not a function" and killed the upgrade.

Extend EventEmitter and emit "finish" then "close" once the handshake has
been written, so those middlewares run to completion instead of crashing.
statusCode is set to 101 so a logger reports the handshake rather than an
undefined status.

Author: Minhal128

packages/engine.io/lib/server.ts

@@ -685,11 +685,17 @@ export abstract class BaseServer extends EventEmitter {
  *
  * @see https://nodejs.org/api/http.html#class-httpserverresponse
  */
-class WebSocketResponse {
+class WebSocketResponse extends EventEmitter {
+  /**
+   * The status code of the handshake response written by the "ws" package.
+   */
+  public statusCode = 101;
+
   constructor(
     readonly req,
     readonly socket: Duplex,
   ) {
+    super();
     // temporarily store the response headers on the req object (see the "headers" event)
     req[kResponseHeaders] = {};
   }
@@ -875,6 +881,11 @@ export class Server extends BaseServer {
 
       // delegate to ws
       this.ws.handleUpgrade(engineRequest, socket, head, (websocket) => {
+        // the handshake response has been written, so middlewares which track the
+        // response lifecycle (loggers, metrics) can now complete
+        res.emit("finish");
+        res.emit("close");
+
         this.onWebSocket(engineRequest, socket, websocket);
       });
     };

packages/engine.io/test/middlewares.js

@@ -249,6 +249,38 @@ describe("middlewares", () => {
     });
   });
 
+  it("should support middlewares that track the response lifecycle (websocket)", function (done) {
+    // the uWebSockets.js server has its own response wrapper, see ResponseWrapper in lib/userver.ts
+    if (process.env.EIO_WS_ENGINE === "uws") {
+      return this.skip();
+    }
+    const engine = listen((port) => {
+      let responseClosed = false;
+
+      engine.use((req, res, next) => {
+        // response loggers such as pino-http register listeners on the response
+        res.on("close", () => {
+          responseClosed = true;
+        });
+        next();
+      });
+
+      const socket = new WebSocket(
+        `ws://localhost:${port}/engine.io/?EIO=4&transport=websocket`,
+      );
+
+      socket.on("open", () => {
+        expect(responseClosed).to.be(true);
+
+        socket.close();
+        if (engine.httpServer) {
+          engine.httpServer.close();
+        }
+        done();
+      });
+    });
+  });
+
   it("should fail on errors (polling)", (done) => {
     const engine = listen((port) => {
       engine.use((req, res, next) => {