docs: def _GetChunk(self):
      """Fetches a chunk corresponding to the current offset."""
  
      found_ref = None
      for ref in self._blob_refs:
        if self._offset >= ref.offset and self._offset < (ref.offset + ref.size):
          found_ref = ref
          break
  
      if not found_ref:
        return None, None
  
      # If self._current_ref == found_ref, then simply return previously found
      # chunk. Otherwise, update self._current_chunk value.
      if self._current_ref != found_ref:
        self._current_ref = found_ref
  
        data = data_store.BLOBS.ReadBlobs([found_ref.blob_id])
        self._current_chunk = data[found_ref.blob_id]
  
      return self._current_chunk, self._current_ref