Connection#open() promise rejection is broken

[daltones]

Do you want to request a feature or report a bug?
Report a bug

What is the current behavior?
When Connection#open() fails, it is emitting an error event that turns into an uncaught exception, breaking the promise rejection handling.

If the current behavior is a bug, please provide the steps to reproduce.

import mongoose from 'mongoose';

const conn = mongoose.createConnection(); // don't open now

async function start() {
  try {
    await conn.open('some wrong connection string');
    // this promise will throw
    // but before that an error event is emitted, leading to an uncaught exception
  } catch (error) {
    console.log('error on open conn');
  }
}

start();

I'm guessing the problem is here:

mongoose/lib/connection.js

Line 316 in 2dd76ab

if (!callback && !promise.$hasHandler) {

That promise.$hasHandler is not valid here, once $hasHandler is a property of MongooseThenable instances (which is not the case there).

This affects Connection#openSet() too.

What is the expected behavior?
While open returns a promise, I expect that any error appears only as the rejection reason and not thrown somewhere else.

Please mention your node.js, mongoose and MongoDB version.
node.js: v7.10.0
mongoose: v4.9.7
MongoDB: v3.4.3

[vkarpov15]

As a workaround, add an on('error') handler.

const mongoose = require('mongoose');

const conn = mongoose.createConnection(); // don't open now

conn.on('error', () => {}); // Add an error handler to prevent the exception

async function start() {
  try {
    await conn.open('some wrong connection string');
    // this promise will throw
    // but before that an error event is emitted, leading to an uncaught exception
  } catch (error) {
    console.log('error on open conn');
  }
}

start();

[vkarpov15]

Fix will be in 4.9.9. Monkey-patching .then() hurts but is necessary