code: def _on_bytes_read(self, num_bytes_read):
          
          self.actual_bytes_read += num_bytes_read
          if self.actual_bytes_read > self.bytes_to_read:
              raise TooLargeChunkDownloadError(self.actual_bytes_read, self.bytes_to_read, self.local_path)
          self.download_context.send_processed_message(num_bytes_read)
docs: Record our progress so we can validate that we receive all the data
          :param num_bytes_read: int: number of bytes we received as part of one chunk