http: make ClientRequest#setTimeout() noop at end

Originally discovered and resolved by @szmarczak.

PR-URL: #25536
Fixes: #25499
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/_http_client.js

@@ -731,6 +731,10 @@ function _deferToConnect(method, arguments_, cb) {
 }
 
 ClientRequest.prototype.setTimeout = function setTimeout(msecs, callback) {
+  if (this._ended) {
+    return this;
+  }
+
   listenSocketTimeout(this);
   msecs = validateTimerDuration(msecs);
   if (callback) this.once('timeout', callback);

test/parallel/test-http-client-set-timeout-after-end.js

@@ -0,0 +1,33 @@
+'use strict';
+
+// Test https://github.com/nodejs/node/issues/25499 fix.
+
+const { mustCall } = require('../common');
+
+const { Agent, createServer, get } = require('http');
+const { strictEqual } = require('assert');
+
+const server = createServer(mustCall((req, res) => {
+  res.end();
+}));
+
+server.listen(0, () => {
+  const agent = new Agent({ keepAlive: true, maxSockets: 1 });
+  const port = server.address().port;
+
+  let socket;
+
+  const req = get({ agent, port }, (res) => {
+    res.on('end', () => {
+      strictEqual(req.setTimeout(0), req);
+      strictEqual(socket.listenerCount('timeout'), 0);
+      agent.destroy();
+      server.close();
+    });
+    res.resume();
+  });
+
+  req.on('socket', (sock) => {
+    socket = sock;
+  });
+});