std::unique_ptr<NowSecondsEnv> env(new NowSecondsEnv);
  uint64 now = Env::Default()->NowSeconds();
  env->SetNowSeconds(now);
  tf_gcs_filesystem::RamFileBlockCache cache(
      8, 32, 1 /* max staleness */, fetcher,
      [&env]() { return env->NowSeconds(); });
  // Read three blocks into the cache, and advance the timestamp by one second

      BlockFetcher;

  RamFileBlockCache(size_t block_size, size_t max_bytes, uint64_t max_staleness,
                    BlockFetcher block_fetcher,
                    std::function<uint64_t()> timer_seconds = TF_NowSeconds)
      : block_size_(block_size),
        max_bytes_(max_bytes),
        max_staleness_(max_staleness),
        block_fetcher_(block_fetcher),
        timer_seconds_(timer_seconds),
        pruning_thread_(nullptr,