copy: comput blob compression on reused blobs based on source MediaType

When copying blobs, if the source LayerInfo array includes MediaType
information, use it to compute the compression that was used for a
source blob, and if we end up reusing a blob with the same digest, use
that computed information.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

copy/copy.go

@@ -810,6 +810,17 @@ func (ic *imageCopier) copyLayers(ctx context.Context) error {
 		srcInfosUpdated = true
 	}
 
+	for i := range srcInfos {
+		if srcInfos[i].CompressionAlgorithm == nil {
+			switch srcInfos[i].MediaType {
+			case manifest.DockerV2Schema2LayerMediaType, imgspecv1.MediaTypeImageLayerGzip:
+				srcInfos[i].CompressionAlgorithm = &compression.Gzip
+			case imgspecv1.MediaTypeImageLayerZstd:
+				srcInfos[i].CompressionAlgorithm = &compression.Zstd
+			}
+		}
+	}
+
 	type copyLayerData struct {
 		destInfo types.BlobInfo
 		diffID   digest.Digest
@@ -1097,6 +1108,11 @@ func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, to
 					Artifact: srcInfo,
 				}
 			}
+
+			if blobInfo.Digest == srcInfo.Digest && blobInfo.CompressionAlgorithm == nil {
+				blobInfo.CompressionOperation = srcInfo.CompressionOperation
+				blobInfo.CompressionAlgorithm = srcInfo.CompressionAlgorithm
+			}
 			return blobInfo, cachedDiffID, nil
 		}
 	}