copy: skip reusingblob if compression algorithm is different

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

copy/copy.go

@@ -1065,12 +1065,18 @@ func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, to
 	// Diffs are needed if we are encrypting an image or trying to decrypt an image
 	diffIDIsNeeded := ic.diffIDsAreNeeded && cachedDiffID == "" || toEncrypt || (isOciEncrypted(srcInfo.MediaType) && ic.c.ociDecryptConfig != nil)
 
+	srcAlgo := ""
+	if srcInfo.CompressionAlgorithm != nil {
+		srcAlgo = srcInfo.CompressionAlgorithm.Name()
+	}
 	// If we already have the blob, and we don't need to compute the diffID, then we don't need to read it from the source.
-	if !diffIDIsNeeded {
+	mustRecompress := srcInfo.CompressionOperation != types.PreserveOriginal || srcAlgo != ic.c.compressionFormat.Name()
+	if !diffIDIsNeeded && !mustRecompress {
 		reused, blobInfo, err := ic.c.dest.TryReusingBlob(ctx, srcInfo, ic.c.blobInfoCache, ic.canSubstituteBlobs)
 		if err != nil {
 			return types.BlobInfo{}, "", errors.Wrapf(err, "Error trying to reuse blob %s at destination", srcInfo.Digest)
 		}
+
 		if reused {
 			logrus.Debugf("Skipping blob %s (already present):", srcInfo.Digest)
 			bar := ic.c.createProgressBar(pool, srcInfo, "blob", "skipped: already exists")