eachAsync does not wait for promises when parallel is greater than the number of records in the cursor

[rrglomsk]

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

What is the current behavior?
For eachAsync, if the 'parallel' option is set to a higher number than the number of records in the cursor, eachAsync will move on without waiting for functions to resolve. (For example, when the number of records in the cursor is 3 and the options are set to { parallel: 4 })

This does not appear to be resolved by the eachAsync fixes made in 5.7.14.

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

Here's a failing test to demonstrate- I just borrowed the setup from test/es-next/asyncIterator.test.es6.js:

'use strict';

const assert = require('assert');
const start = require('../common');

const mongoose = start.mongoose;

describe('eachAsync', function() {
  let db;
  let Movie;

  before(async function() {
    db = await start();

    const schema = new mongoose.Schema({ name: String });
    Movie = db.model('gh6737_Movie', schema);

    await Movie.create([
      { name: 'Kickboxer' },
      { name: 'Ip Man' },
      { name: 'Enter the Dragon' }
    ]);
  });

  after(function(done) {
    db.close(done);
  });

  it('waits for all promises to resolve when the parallel setting surpasses the number of records in the cursor', async function() {
    var a = false;

    async function test() {
      await new Promise((resolve) => setTimeout(resolve, 100));
      a = true;
    }

    await Movie.find().cursor().eachAsync(test, { parallel: 4 })
    assert.equal(a, true);
  });

Note this test will pass when parallel is less than or equal to 3.

What is the expected behavior?
eachAsync will wait for all functions to resolve, regardless of number of records in the cursor and what the parallel count is set to.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node.js: 12.13.1
Mongoose: 5.7.14
MongoDB: 4.2.1

[rrglomsk]

https://github.com/Automattic/mongoose/blob/master/lib/helpers/cursor/eachAsync.js#L71

Looks like this might be the culprit at a glance- should it happen before done() is called?
Otherwise when there's a null doc in the next function (due to there being a parallel > record count), there are "no results in progress" and it moves on to the final callback.

[vkarpov15]

Fixed. @rrglomsk 's solution turned out to be correct, it just proved very difficult to repro this without using a cursor. I couldn't repro this in test/helpers/cursor.eachAsync.test.js, needed to use a real cursor as opposed to a stub function.

[rrglomsk]

Awesome, thank you for taking a look!! 😄