if (!counter.WaitFor(std::chrono::seconds(10))) {
      // This avoids having the test time out, which is harder to debug.
      TF_SetStatus(status, TF_FAILED_PRECONDITION,
                   "desired concurrency not reached");
      return -1;
    }
    memset(buffer, 'x', n);
    TF_SetStatus(status, TF_OK, "");
    return n;
  };
  const int block_size = 8;