docs: def _UploadChunk(self, chunk):
      """Uploads a single chunk to the transfer store flow.
  
      Args:
        chunk: A chunk to upload.
  
      Returns:
        A `BlobImageChunkDescriptor` object.
      """
      blob = _CompressedDataBlob(chunk)
  
      self._action.ChargeBytesToSession(len(chunk.data))
      self._action.SendReply(blob, session_id=self._TRANSFER_STORE_SESSION_ID)
  
      return rdf_client_fs.BlobImageChunkDescriptor(
          digest=hashlib.sha256(chunk.data).digest(),
          offset=chunk.offset,
          length=len(chunk.data))