code: func (u *ConcurrentUploader) Close() error {
  	u.wg.Wait()
  	close(u.waitc) // Sanity check that we don't do any more uploading.
  	return u.err
  }
docs: // Close waits for any pending uploads (and associated done callbacks) to
  // complete, and returns the first encountered error if any.
  // Uploader cannot be used once it is closed.