events: improve max listeners warning

This adds the constructor name of the event target to the emitted
warning. Right now it's difficult to identify where the leak is
actually coming from and having some further information about the
source will likely help to identify the source.

PR-URL: https://github.com/nodejs/node/pull/27694
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/events.js

@@ -31,6 +31,10 @@ const {
   ERR_UNHANDLED_ERROR
 } = require('internal/errors').codes;
 
+const {
+  inspect
+} = require('internal/util/inspect');
+
 function EventEmitter() {
   EventEmitter.init.call(this);
 }
@@ -253,8 +257,8 @@ function _addListener(target, type, listener, prepend) {
       // eslint-disable-next-line no-restricted-syntax
       const w = new Error('Possible EventEmitter memory leak detected. ' +
                           `${existing.length} ${String(type)} listeners ` +
-                          'added. Use emitter.setMaxListeners() to ' +
-                          'increase limit');
+                          `added to ${inspect(target, { depth: -1 })}. Use ` +
+                          'emitter.setMaxListeners() to increase limit');
       w.name = 'MaxListenersExceededWarning';
       w.emitter = target;
       w.type = type;

test/parallel/test-event-emitter-max-listeners-warning-for-null.js

@@ -15,7 +15,8 @@ process.on('warning', common.mustCall((warning) => {
   assert.strictEqual(warning.emitter, e);
   assert.strictEqual(warning.count, 2);
   assert.strictEqual(warning.type, null);
-  assert.ok(warning.message.includes('2 null listeners added.'));
+  assert.ok(warning.message.includes(
+    '2 null listeners added to [EventEmitter].'));
 }));
 
 e.on(null, () => {});

test/parallel/test-event-emitter-max-listeners-warning-for-symbol.js

@@ -17,7 +17,8 @@ process.on('warning', common.mustCall((warning) => {
   assert.strictEqual(warning.emitter, e);
   assert.strictEqual(warning.count, 2);
   assert.strictEqual(warning.type, symbol);
-  assert.ok(warning.message.includes('2 Symbol(symbol) listeners added.'));
+  assert.ok(warning.message.includes(
+    '2 Symbol(symbol) listeners added to [EventEmitter].'));
 }));
 
 e.on(symbol, () => {});

test/parallel/test-event-emitter-max-listeners-warning.js

@@ -6,7 +6,14 @@ const common = require('../common');
 const events = require('events');
 const assert = require('assert');
 
-const e = new events.EventEmitter();
+class FakeInput extends events.EventEmitter {
+  resume() {}
+  pause() {}
+  write() {}
+  end() {}
+}
+
+const e = new FakeInput();
 e.setMaxListeners(1);
 
 process.on('warning', common.mustCall((warning) => {
@@ -15,7 +22,8 @@ process.on('warning', common.mustCall((warning) => {
   assert.strictEqual(warning.emitter, e);
   assert.strictEqual(warning.count, 2);
   assert.strictEqual(warning.type, 'event-type');
-  assert.ok(warning.message.includes('2 event-type listeners added.'));
+  assert.ok(warning.message.includes(
+    '2 event-type listeners added to [FakeInput].'));
 }));
 
 e.on('event-type', () => {});