/// A list iterator pointing to the entry's position in the LRU list.
    std::list<std::string>::iterator lru_iterator;
  };

  bool LookupLocked(const std::string& key, T* value)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
    auto it = cache_.find(key);
    if (it == cache_.end()) {
      return false;
    }
    lru_list_.erase(it->second.lru_iterator);
    if (timer_seconds_() - it->second.timestamp > max_age_) {

      ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);

  /// Remove the block `entry` from the block map and LRU list, and update the
  /// cache size accordingly.
  void RemoveBlock(BlockMap::iterator entry) ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);

  /// The cache pruning thread that removes files with expired blocks.
  std::unique_ptr<TF_Thread, std::function<void(TF_Thread*)>> pruning_thread_;