fix(db): throw readable error when creating _id with background: true

[vkarpov15]

My least favorite bug strikes again: createIndex() falls back to inserting an index doc and swallows the original error. Below is a minimal repro script with mongodb@3.0.9:

const { MongoClient } = require('mongodb');

run().catch(error => console.error(error.stack));

async function run() {
  const client = await MongoClient.connect('mongodb://localhost:27017/test');

  const db = client.db();

  await db.collection('Test2').ensureIndex({ _id: 1 }, { background: true });

  console.log('Done');
}

This throws a very unhelpful error:

$ node gh-6109.js 
Error: cyclic dependency detected
    at serializeObject (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:333:34)
    at serializeInto (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:937:17)
    at serializeObject (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:347:18)
    at serializeInto (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:937:17)
    at serializeObject (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:347:18)
    at serializeInto (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:937:17)
    at serializeObject (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:347:18)
    at serializeInto (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:937:17)
    at serializeObject (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:347:18)
    at serializeInto (/home/val/Workspace/MongoDB/troubleshoot-mongoose/node_modules/bson/lib/bson/parser/serializer.js:937:17)

Admittedly, trying to create an index on _id is impossible because MongoDB creates one by default, but plenty of people do this in mongoose, so a lot of people are getting this baffling "cyclic dependency detected" error (see Automattic/mongoose#6109). Unfortunately I can't fix this issue from mongoose, so can this get into 3.0.x so I can close out Automattic/mongoose#6109?

[vkarpov15]

Also, a related issue: what do you think about adding a toBSON() method to the ClientSession class? This cyclic dependency is because bson chokes on serializing a session.

[mbroadst]

@vkarpov15 thanks for this! I agree we should throw a toBSON on ClientSession, if not just to have it throw an exception. I made NODE-1518 to track this, perhaps one of our interns can even pick it up today.