reconnect helper for change streams

[vkarpov15]

Re: #6741, change streams currently fail when there's a replica set failover. We should have some mechanism in place to handle this case. @mbroadst what are your thoughts?

[mbroadst]

@vkarpov15 change streams were designed explicitly to support handling failover, see: https://github.com/mongodb/node-mongodb-native/blob/master/lib/change_stream.js#L348-L379. Perhaps you are running into a non-resumable error case? We would need more information about the specific error scenario, and ideally a reproducible case would help us resolve this very quickly.

[vkarpov15]

@mbroadst I can repro this consistently in mongoose by running rs.stepDown() in the shell while the below script is running:

const assert = require('assert');
const mongoose = require('mongoose');
mongoose.set('debug', true);

const GITHUB_ISSUE = `gh6741`;
const connectionString = `mongodb://localhost:27017/${ GITHUB_ISSUE }`;
const { Schema } = mongoose;

run().then(() => console.log('done')).catch(error => console.error(error.stack));

async function run() {
  await mongoose.connect(connectionString, { useNewUrlParser: true, replicaSet: 'rs' });
  await mongoose.connection.dropDatabase();

  await mongoose.connection.createCollection('Foo');
  
  const M = mongoose.model('Foo', new Schema({ name: String }), 'Foo');
  
  let changeStream = M.watch().on('change', change => console.log(change));
  console.log('Watching...');
}

Do you need a driver-only script or is this enough to get you started?

[mbroadst]

@vkarpov15 that's enough for me, and yeah it looks like there's a bug here. Our resume logic is only presently tested when using the change stream as a cursor, not as a stream. I made NODE-1617 to track this, we'll get on a fix ASAP.

[vkarpov15]

Should be fixed by #6920