Fetches a chunk corresponding to the current offset.

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