eachAsync not waiting for all promises when parallel and populate is used

[simllll]

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

What is the current behavior?
when using eachAsync with parallel option set to 10 and having a populate on the query, mongoose returns even before all awaits are finished.

see test script, output:

a
a
a
a
b
DONE <-- this sohuld be defintily after all 'b's.. 
b
b
b
b
b
b
b
b
b

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

(async () => {
	const jobsRes = await DBJobModel.find({})
		.populate('_location _field _type _company', '-searchQueries') // <-- need to populate some stuff, it works when uncommenting this line.
		.lean()
		.limit(10)
		.cursor();

	await jobsRes.eachAsync(
		async jobOrg => {
			console.log('a');
			await new Promise((resolve) => setTimeout(resolve, 1000));
			console.log('b');
		}
	, { parallel: 10 });

	console.log('DONE');
})();

What is the expected behavior?
eachAsync needs to wait for all promises

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
mongoose 5.7.11
node v12.13.0
mongodb 4.2.1

[lineus]

Here's a repro script for this one:

#!/usr/bin/env node
'use strict';

const mongoose = require('mongoose');
const { Schema, connection} = mongoose;
const DB = '8352';
const URI = `mongodb://localhost:27017/${DB}`;
const OPTS = { useNewUrlParser: true, useUnifiedTopology: true };

const schema = new Schema({
  name: String
});

const otherSchema = new Schema({
  tests: [Schema.Types.ObjectId]
});

const Test = mongoose.model('test', schema);
const Other = mongoose.model('other', otherSchema);

function makeTests() {
  return Array.from({ length: 10 }).map((_, i) => {
    return new Test({
      name: `name${i}`
    });
  });
}

const tests1 = makeTests();
const tests2 = makeTests();

const other1 = new Other({ others: tests1.map(t => t._id) });
const other2 = new Other({ others: tests2.map(t => t._id) });

async function run() {
  await mongoose.connect(URI, OPTS);
  await connection.dropDatabase();
  await Test.create(tests1.concat(tests2));
  await Other.create([other1, other2]);
  const others = await Other.find({})
    .populate('tests')
    .lean()
    .limit(10)
    .cursor();
  await others.eachAsync(
    async () => {
      console.log('a');
      await new Promise((resolve) => setTimeout(resolve, 1000));
      console.log('b');
    }
    , { parallel: 10 });
  console.log('done');
  await connection.close();
}

run();

output:

issues: ./8352.js 
a
a
done
b
b
issues: 

[simllll]

Thanks a lot :)

[vkarpov15]

No worries, thanks for pointing out this issue! Clearly our tests weren't quite rigorous enough.