There does not appear to be a way to determine whether a mongoc_collection_update failed to locate any documents that satisfy the query. I would expect this is an error condition in many applications (in my case it revealed a thread race issue).

I have found 2 work arounds. One is to put the update into a bulk operation and look at the reply from mongoc_bulk_operation_execute(). The other is to use mongoc_collection_find_and_modify() and request at least one field return. Any comment on which is more efficient?