fix(node:http): fall back upgrade requests to 'request' event when no 'upgrade' listener

When a WebSocket/upgrade request is sent to an `http.createServer()` server
that has no 'upgrade' event listener, Node.js falls back to emitting a
'request' event. Bun was unconditionally emitting the 'upgrade' event, which
was silently dropped when no listener existed.

Check `server.listenerCount("upgrade")` before deciding to emit the upgrade
event, and fall through to normal request handling when no listeners exist.

Closes #26924

Co-Authored-By: Claude <noreply@anthropic.com>

src/js/node/_http_server.ts

@@ -582,7 +582,8 @@ Server.prototype[kRealListen] = function (tls, port, host, socketPath, reusePort
 
         socket[kRequest] = http_req;
         const is_upgrade = http_req.headers.upgrade;
-        if (!is_upgrade) {
+        const hasUpgradeListeners = is_upgrade && server.listenerCount("upgrade") > 0;
+        if (!hasUpgradeListeners) {
           if (canUseInternalAssignSocket) {
             // ~10% performance improvement in JavaScriptCore due to avoiding .once("close", ...) and removing a listener
             assignSocketInternal(http_res, socket);
@@ -601,7 +602,7 @@ Server.prototype[kRealListen] = function (tls, port, host, socketPath, reusePort
           http_res.writeHead(503);
           http_res.end();
           socket.destroy();
-        } else if (is_upgrade) {
+        } else if (hasUpgradeListeners) {
           server.emit("upgrade", http_req, socket, kEmptyBuffer);
           if (!socket._httpMessage) {
             if (canUseInternalAssignSocket) {

test/regression/issue/26924.test.ts

@@ -0,0 +1,111 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+test("node:http server falls back upgrade request to 'request' event when no 'upgrade' listener", async () => {
+  await using proc = Bun.spawn({
+    cmd: [
+      bunExe(),
+      "-e",
+      `
+const http = require('node:http');
+
+const server = http.createServer();
+const events = [];
+
+server.on('request', (req, res) => {
+  events.push('request');
+  res.end();
+});
+
+// No 'upgrade' listener registered
+
+server.listen(0, function() {
+  const port = this.address().port;
+  // Send a request with Upgrade header using http module
+  const req = http.request({
+    hostname: 'localhost',
+    port,
+    path: '/',
+    method: 'GET',
+    headers: {
+      'Connection': 'Upgrade',
+      'Upgrade': 'websocket',
+    },
+  }, (res) => {
+    events.push('response');
+    res.resume();
+    res.on('end', () => {
+      console.log(JSON.stringify(events));
+      server.close();
+    });
+  });
+  req.end();
+});
+`,
+    ],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  const events = JSON.parse(stdout.trim());
+  expect(events).toEqual(["request", "response"]);
+  expect(exitCode).toBe(0);
+});
+
+test("node:http server emits 'upgrade' event when listener is registered", async () => {
+  await using proc = Bun.spawn({
+    cmd: [
+      bunExe(),
+      "-e",
+      `
+const http = require('node:http');
+
+const server = http.createServer();
+const events = [];
+
+server.on('request', (req, res) => {
+  events.push('request');
+  res.end();
+});
+
+server.on('upgrade', (req, socket) => {
+  events.push('upgrade');
+  socket.end();
+});
+
+server.listen(0, function() {
+  const port = this.address().port;
+  const req = http.request({
+    hostname: 'localhost',
+    port,
+    path: '/',
+    method: 'GET',
+    headers: {
+      'Connection': 'Upgrade',
+      'Upgrade': 'websocket',
+    },
+  });
+  req.on('error', () => {});
+  req.end();
+  // Give the server time to process
+  setTimeout(() => {
+    console.log(JSON.stringify(events));
+    server.close();
+  }, 500);
+});
+`,
+    ],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  const events = JSON.parse(stdout.trim());
+  expect(events).toEqual(["upgrade"]);
+  expect(exitCode).toBe(0);
+});