/// be fetched from the backing filesystem. This callback is provided when the
  /// cache is constructed. It returns total bytes read ( -1 in case of errors
  /// ). The `status` should be `TF_OK` as long as the read from the remote
  /// filesystem succeeded (similar to the semantics of the read(2) system
  /// call).
  typedef std::function<int64_t(const std::string& filename, size_t offset,