code: protected synchronized ExecutorService getExecutorService(String stageName) {
          if (executorServiceFactory != null) {
              return executorServiceFactory.getExecutorService(stageName);
          }
          // TODO: Use a default executor? Drive on current thread? Throw exception?
          return null;
      }
docs: Get the named {@link java.util.concurrent.ExecutorService} responsible
  for hosting the stage from the {@link WorkStageManager}.
  
  @param stageName
  the name of the stage
  
  @return the {@code ExecutorService} to use for event delivery or {@code null} if the executor cannot be acquired