auto fetcher = [&counter](const string& filename, size_t offset, size_t n,
                            char* buffer, TF_Status* status) -> int64_t {
    counter.DecrementCount();
    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;