fix(eio): make the upgrade response an EventEmitter

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) => {