fix(bulk): allow upsert with empty update when using bulkWrite()

[vkarpov15]

Description

In v3.5 bulkWrite() could do an updateOne with an empty update, so long as upsert was set. For example, the below script succeeds in v3.5.8 but throws a "Update document requires atomic operators" error in v3.6.

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

void async function() {
  const client = await MongoClient.connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true,
    useUnifiedTopology: true
  });

  const db = client.db();
  
  await db.collection('Test').bulkWrite([{
    updateOne: { filter: { name: 'test' }, update: {}, upsert: true }
  }]);

  console.log('Done');
  process.exit(0);
}();

What changed?

Added a check to allow the above script to pass

Are there any files to ignore?

Nope

[mbroadst]

Hey @vkarpov15 , I think this behavior change is desired. Can't you refactor the code to use an atomic operator:

  await db.collection('Test').bulkWrite([{
    updateOne: { filter: { name: 'test' }, update: { $set: {} }, upsert: true }
  }]);

?

[vkarpov15]

I could do that, the only problem is that the server throws errors when you do an empty $set. The below script:

const { MongoClient, ObjectId } = require('mongodb');
  
void async function() {
  const client = await MongoClient.connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true,
    useUnifiedTopology: true
  });

  const db = client.db();

  await db.collection('Test').bulkWrite([{
    updateOne: { filter: { name: 'test' }, update: { $set: {} }, upsert: true }
  }]);

  console.log('Done');
  process.exit(0);
}();

Throws the below error:

$ node ./test.js 
(node:10277) UnhandledPromiseRejectionWarning: BulkWriteError: '$set' is empty. You must specify a field like so: {$set: {<field>: ...}}
    at OrderedBulkOperation.handleWriteError (/troubleshoot-mongoose/node_modules/mongodb/lib/bulk/common.js:1213:9)
    at resultHandler (/troubleshoot-mongoose/node_modules/mongodb/lib/bulk/common.js:519:23)

Is this behavior defined in a spec somewhere? If not, I'd like to ask that we not go down the route of throwing errors on empty updates. It's already tricky to work around the empty $set issue in the server: https://jira.mongodb.org/browse/SERVER-12266, and this change breaks our workaround.

[mbroadst]

@vkarpov15 oops, good catch! Let me get back to you shortly on this one

[mbroadst]

@vkarpov15 I think we can close this now based on the discussion in DRIVERS-1359?

[vkarpov15]

@mbroadst yep! Thanks for your patience in explaining this change