Status processUsers(OperationContext* txn,
                        AuthorizationManager* authzManager,
                        StringData usersCollName,
                        StringData db,
                        bool drop) {
        // When the "drop" argument has been provided, we use this set to store the users
        // that are currently in the system, and remove from it as we encounter
        // same-named users in the collection we are restoring from.  Once we've fully
        // moved over the temp users collection into its final location, we drop
        // any users that previously existed there but weren't in the temp collection.
        // This is so that we can completely replace the system.users
        // collection with the users from the temp collection, without removing all
        // users at the beginning and thus potentially locking ourselves out by having
        // no users in the whole system for a time.
        unordered_set<UserName> usersToDrop;

        if (drop) {
            // Create map of the users currently in the DB
            BSONObj query =
                db.empty() ? BSONObj() : BSON(AuthorizationManager::USER_DB_FIELD_NAME << db);
            BSONObj fields = BSON(AuthorizationManager::USER_NAME_FIELD_NAME
                                  << 1
                                  << AuthorizationManager::USER_DB_FIELD_NAME
                                  << 1);

            Status status =
                queryAuthzDocument(txn,
                                   AuthorizationManager::usersCollectionNamespace,
                                   query,
                                   fields,
                                   stdx::bind(&CmdMergeAuthzCollections::extractAndInsertUserName,
                                              &usersToDrop,
                                              stdx::placeholders::_1));
            if (!status.isOK()) {
                return status;
            }
        }

        Status status = queryAuthzDocument(
            txn,
            NamespaceString(usersCollName),
            db.empty() ? BSONObj() : BSON(AuthorizationManager::USER_DB_FIELD_NAME << db),
            BSONObj(),
            stdx::bind(&CmdMergeAuthzCollections::addUser,
                       txn,
                       authzManager,
                       db,
                       drop,
                       &usersToDrop,
                       stdx::placeholders::_1));
        if (!status.isOK()) {
            return status;
        }

        if (drop) {
            long long numRemoved;
            for (unordered_set<UserName>::iterator it = usersToDrop.begin();
                 it != usersToDrop.end();
                 ++it) {
                const UserName& userName = *it;
                audit::logDropUser(Client::getCurrent(), userName);
                status = removePrivilegeDocuments(txn,
                                                  BSON(AuthorizationManager::USER_NAME_FIELD_NAME
                                                       << userName.getUser().toString()
                                                       << AuthorizationManager::USER_DB_FIELD_NAME
                                                       << userName.getDB().toString()),
                                                  &numRemoved);
                if (!status.isOK()) {
                    return status;
                }
                dassert(numRemoved == 1);
            }
        }

        return Status::OK();
    }