In SchedulerWorkerPoolImpl::JoinForTesting(), a copy of workers_ is created before SchedulerWorker::JoinForTesting() is called on individual workers. 

It is assumed that workers_ does not change after the copy is created (i.e. no workers are created & added to workers_ and no workers_ are removed).

Instead, there should be a flag that is set within SchedulerWorkerPoolImpl::JoinForTesting() that verifies that no changes are made to workers_.