docs: def StoreStat(self, responses):
      """Stores stat entry in the flow's state."""
      index = responses.request_data["index"]
      if not responses.success:
        self.Log("Failed to stat file: %s", responses.status)
        # Report failure.
        self._FileFetchFailed(index, responses.request_data["request_name"])
        return
  
      tracker = self.state.pending_hashes[index]
      tracker["stat_entry"] = responses.First()