gcs::Client* gcs_client;  // not owned
  TempFile outfile;
  bool sync_need;
  // `offset` tells us how many bytes of this file are already uploaded to
  // server. If `offset == -1`, we always upload the entire temporary file.
  int64_t offset;
} GCSFile;

static void SyncImpl(const std::string& bucket, const std::string& object,
                     int64_t* offset, TempFile* outfile,