From aabfe336f461824d0448bdb07d26225027f91b80 Mon Sep 17 00:00:00 2001
From: nabsei <nsahrane@hotmail.com>
Date: Wed, 15 Jul 2026 01:58:42 +0200
Subject: [PATCH] fix(eio): expose EventEmitter interface on WebSocket upgrade
 response
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WebSocketResponse (the fake response object passed to middlewares
during a WebSocket upgrade) did not implement EventEmitter, so any
middleware calling res.on(...) — e.g. pino-http listening for "close"
to know when the request finished — crashed with "res.on is not a
function".

Make WebSocketResponse extend EventEmitter and emit "close" when the
underlying socket closes, and "finish" when end() is called, mirroring
the events a real http.ServerResponse emits.

Fixes #5072

Co-authored-by: Claude <noreply@anthropic.com>
---
 packages/engine.io/lib/server.ts       |  7 ++++++-
 packages/engine.io/test/middlewares.js | 23 +++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/packages/engine.io/lib/server.ts b/packages/engine.io/lib/server.ts
index c4f2c30d4c..293c741974 100644
--- a/packages/engine.io/lib/server.ts
+++ b/packages/engine.io/lib/server.ts
@@ -669,13 +669,17 @@ export abstract class BaseServer extends EventEmitter {
  *
  * @see https://nodejs.org/api/http.html#class-httpserverresponse
  */
-class WebSocketResponse {
+class WebSocketResponse extends EventEmitter {
   constructor(
     readonly req,
     readonly socket: Duplex,
   ) {
+    super();
     // temporarily store the response headers on the req object (see the "headers" event)
     req[kResponseHeaders] = {};
+    // some middlewares (like pino-http) rely on the "close" event to know when the response is done, which the
+    // underlying socket does not expose by default
+    socket.once("close", () => this.emit("close"));
   }
 
   public setHeader(name: string, value: any) {
@@ -695,6 +699,7 @@ class WebSocketResponse {
   public writeHead() {}
 
   public end() {
+    this.emit("finish");
     // we could return a proper error code, but the WebSocket client will emit an "error" event anyway.
     this.socket.destroy();
   }
diff --git a/packages/engine.io/test/middlewares.js b/packages/engine.io/test/middlewares.js
index 60e1cc6b22..6e97711999 100644
--- a/packages/engine.io/test/middlewares.js
+++ b/packages/engine.io/test/middlewares.js
@@ -56,6 +56,29 @@ describe("middlewares", () => {
     });
   });
 
+  it("should expose EventEmitter methods on the response object during upgrade (regression for pino-http)", (done) => {
+    const engine = listen((port) => {
+      engine.use((req, res, next) => {
+        expect(res.on).to.be.a("function");
+        res.on("close", () => {
+          if (engine.httpServer) {
+            engine.httpServer.close();
+          }
+          done();
+        });
+        next();
+      });
+
+      const socket = new WebSocket(
+        `ws://localhost:${port}/engine.io/?EIO=4&transport=websocket`,
+      );
+
+      socket.on("open", () => {
+        socket.close();
+      });
+    });
+  });
+
   it("should apply all middlewares in order", (done) => {
     const engine = listen((port) => {
       let count = 0;