Function MakePrimedThreadPool

cpp/src/arrow/acero/task_util_test.cc:46–69

\brief Create a thread pool and start all threads By default a thread pool will not create threads until they are actually needed. This can make it a bit difficult to reproduce certain issues. This creates a thread pool and then makes sure the threads are actually created before returning it.

/// then makes sure the threads are actually created before
/// returning it.
Result<std::shared_ptr<ThreadPool>> MakePrimedThreadPool(int num_threads) {
  ARROW_ASSIGN_OR_RAISE(std::shared_ptr<ThreadPool> thread_pool,
                        ThreadPool::Make(num_threads));
  int num_threads_running = 0;
  std::mutex mutex;
  std::condition_variable thread_gate;
  std::condition_variable primer_gate;
  for (int i = 0; i < num_threads; i++) {
    // This shouldn't fail and, if it fails midway, we will have some threads
    // still running if we do RETURN_NOT_OK so lets do ABORT_NOT_OK
    ABORT_NOT_OK(thread_pool->Spawn([&] {
      std::unique_lock<std::mutex> lk(mutex);
      num_threads_running++;
      primer_gate.notify_one();
      thread_gate.wait(lk);
    }));
  }
  std::unique_lock<std::mutex> primer_lock(mutex);
  primer_gate.wait(primer_lock, [&] { return num_threads_running == num_threads; });
  thread_gate.notify_all();
  primer_lock.unlock();
  thread_pool->WaitForIdle();
  return thread_pool;
}

Status SlowTaskImpl(std::size_t, int64_t) {
  SleepABit();