func (d *Destination) TryReusingBlob(ctx context.Context, info types.BlobInfo, cache types.BlobInfoCache, canSubstitute bool) (bool, types.BlobInfo, error) {
	if info.Digest == "" {
		return false, types.BlobInfo{}, errors.Errorf("Can not check for a blob with unknown digest")
	}
	if blob, ok := d.blobs[info.Digest]; ok {
		return true, types.BlobInfo{Digest: info.Digest, Size: blob.Size}, nil
	}
	return false, types.BlobInfo{}, nil
}
func (d *Destination) TryReusingBlob(ctx context.Context, info types.BlobInfo, cache types.BlobInfoCache, canSubstitute bool) (bool, types.BlobInfo, error) {
	if info.Digest == "" {
		return false, types.BlobInfo{}, errors.Errorf("Can not check for a blob with unknown digest")
	}
	if blob, ok := d.blobs[info.Digest]; ok {
		return true, types.BlobInfo{Digest: info.Digest, Size: blob.Size}, nil
	}
	return false, types.BlobInfo{}, nil
}
// TryReusingBlob checks whether the transport already contains, or can efficiently reuse, a blob, and if so, applies it to the current destination
// (e.g. if the blob is a filesystem layer, this signifies that the changes it describes need to be applied again when composing a filesystem tree).
// info.Digest must not be empty.
// If canSubstitute, TryReusingBlob can use an equivalent equivalent of the desired blob; in that case the returned info may not match the input.
// If the blob has been succesfully reused, returns (true, info, nil); info must contain at least a digest and size.
// If the transport can not reuse the requested blob, TryReusingBlob returns (false, {}, nil); it returns a non-nil error only on an unexpected failure.
// May use and/or update cache.