From c4375f8192128d8dd480750445a9124736552f93 Mon Sep 17 00:00:00 2001
From: Ruben Bridgewater <ruben@bridgewater.de>
Date: Tue, 14 May 2019 16:45:15 +0200
Subject: [PATCH] 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.
---
 lib/events.js                                        |  8 ++++++--
 ...t-event-emitter-max-listeners-warning-for-null.js |  3 ++-
 ...event-emitter-max-listeners-warning-for-symbol.js |  3 ++-
 .../test-event-emitter-max-listeners-warning.js      | 12 ++++++++++--
 4 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/lib/events.js b/lib/events.js
index 3deb15547918..438c8f830624 100644
--- a/lib/events.js
+++ b/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;
diff --git a/test/parallel/test-event-emitter-max-listeners-warning-for-null.js b/test/parallel/test-event-emitter-max-listeners-warning-for-null.js
index 2929e1c70932..40b5f798cbad 100644
--- a/test/parallel/test-event-emitter-max-listeners-warning-for-null.js
+++ b/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, () => {});
diff --git a/test/parallel/test-event-emitter-max-listeners-warning-for-symbol.js b/test/parallel/test-event-emitter-max-listeners-warning-for-symbol.js
index 51c31ba0cf2f..c27b38c25234 100644
--- a/test/parallel/test-event-emitter-max-listeners-warning-for-symbol.js
+++ b/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, () => {});
diff --git a/test/parallel/test-event-emitter-max-listeners-warning.js b/test/parallel/test-event-emitter-max-listeners-warning.js
index 0be7fc84bce1..3d047766b38f 100644
--- a/test/parallel/test-event-emitter-max-listeners-warning.js
+++ b/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', () => {});