From e6f6d4475cc3ec76aa7da990e5ba4f3a8a4fc298 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Wed, 28 Feb 2024 14:49:22 -0500
Subject: [PATCH] fix(connection): avoid unhandled error on createConnection()
 if on('error') handler registered

Fix #14377
---
 lib/connection.js       | 24 ++++++++++++++++++++++++
 test/connection.test.js | 15 +++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/lib/connection.js b/lib/connection.js
index 542bdf3c572..05ff52461b0 100644
--- a/lib/connection.js
+++ b/lib/connection.js
@@ -829,6 +829,30 @@ Connection.prototype.openUri = async function openUri(uri, options) {
   return this;
 };
 
+/*!
+ * Treat `on('error')` handlers as handling the initialConnection promise
+ * to avoid uncaught exceptions when using `on('error')`. See gh-14377.
+ */
+
+Connection.prototype.on = function on(event, callback) {
+  if (event === 'error' && this.$initialConnection) {
+    this.$initialConnection.catch(() => {});
+  }
+  return EventEmitter.prototype.on.call(this, event, callback);
+};
+
+/*!
+ * Treat `once('error')` handlers as handling the initialConnection promise
+ * to avoid uncaught exceptions when using `on('error')`. See gh-14377.
+ */
+
+Connection.prototype.once = function on(event, callback) {
+  if (event === 'error' && this.$initialConnection) {
+    this.$initialConnection.catch(() => {});
+  }
+  return EventEmitter.prototype.once.call(this, event, callback);
+};
+
 /*!
  * ignore
  */
diff --git a/test/connection.test.js b/test/connection.test.js
index 9ea81e356d0..3d1170838cf 100644
--- a/test/connection.test.js
+++ b/test/connection.test.js
@@ -916,6 +916,21 @@ describe('connections:', function() {
     assert.equal(err.name, 'MongooseServerSelectionError');
   });
 
+  it('avoids unhandled error on createConnection() if error handler registered (gh-14377)', async function() {
+    const opts = {
+      serverSelectionTimeoutMS: 100
+    };
+    const uri = 'mongodb://baddomain:27017/test';
+
+    const conn = mongoose.createConnection(uri, opts);
+    await new Promise(resolve => {
+      conn.on('error', err => {
+        assert.equal(err.name, 'MongoServerSelectionError');
+        resolve();
+      });
+    });
+  });
+
   it('`watch()` on a whole collection (gh-8425)', async function() {
     this.timeout(10000);
     if (!process.env.REPLICA_SET) {