FINISHED,
    ERROR,
  };

  /// \brief A block of a file.
  ///
  /// A file block consists of the block data, the block's current position in
  /// the LRU cache, the timestamp (seconds since epoch) at which the block
  /// was cached, a coordination lock, and state & condition variables.
  ///
  /// Thread safety:
  /// The iterator and timestamp fields should only be accessed while holding