Summary

ChangeStream::createPipelineOptionsDoc() may return nil and set cs.err if failed.

But the caller ChangeStream::executeOperation() does not handle cs.err != nil condition or nil return.

This may lead to broken $changeStream BSON command, and then get an error from mongo server : "invalid bson type in element with field name 'rsor' in object with unknown _id "

My develop environment is go-driver v1.6.0, and I comfirmed this bug in v1.9.1 and  master.

My MongoDB server version is MongoDB 4.0

How to Reproduce

1. New a Collection with user-defined bson.registry in a golang project, but the user-defined bson.registry has some bugs to cause transformBsoncoreDocument() failure (Maybe there are other cases to cause this failure)
2. Watch a collection in golang project, and restart MongoDB server to trigger change stream resume
3. The bson.registry bugs lead to transformBsoncoreDocument() failure in ChangeStream::createPipelineOptionsDoc(), and return nil after set cs.err (Maybe there are other cases that lead to  transformBsoncoreDocument() failure)
4. ChangeStream::executeOperation() ignores the cs.err, and continue to build a broken pipeline BSON
5. The broken pipeline BSON command is sent to MongoDB Server, and get "invalid bson" response

Additional Background

ChangeStream::createPipelineOptionsDoc() error is already handled in ChangeStream::buildPipelineSlice(), see:  https://github.com/mongodb/mongo-go-driver/blob/v1.9.1/mongo/change_stream.go#L366-L369

But ChangeStream::executeOperation() don't, see:  https://github.com/mongodb/mongo-go-driver/blob/v1.9.1/mongo/change_stream.go#L249