code: func (s *Snapshotter) SetBlob(ctx context.Context, key string, diffID, blobsum digest.Digest) error {
  	info, err := s.opt.Content.Info(ctx, blobsum)
  	if err != nil {
  		return err
  	}
  	if _, ok := info.Labels["containerd.io/uncompressed"]; !ok {
  		labels := map[string]string{
  			"containerd.io/uncompressed": diffID.String(),
  		}
  		if _, err := s.opt.Content.Update(ctx, content.Info{
  			Digest: blobsum,
  			Labels: labels,
  		}, "labels.containerd.io/uncompressed"); err != nil {
  			return err
  		}
  	}
  	md, _ := s.opt.MetadataStore.Get(key)
  
  	v, err := metadata.NewValue(DiffPair{DiffID: diffID, Blobsum: blobsum})
  	if err != nil {
  		return err
  	}
  	v.Index = index(blobsum)
  
  	return md.Update(func(b *bolt.Bucket) error {
  		return md.SetValue(b, blobKey, v)
  	})
  }
docs: // Validates that there is no blob associated with the snapshot.
  // Checks that there is a blob in the content store.
  // If same blob has already been set then this is a noop.