allow copying and conversion from v2s2 to oci with zstd

Signed-off-by: Luca Guerra <[email protected]>

Author: LucaGuerra

image/copy/compression.go

@@ -27,9 +27,10 @@ var (
 	// expectedBaseCompressionFormats is used to check if a blob with a specified media type is compressed
 	// using the algorithm that the media type says it should be compressed with
 	expectedBaseCompressionFormats = map[string]*compressiontypes.Algorithm{
-		imgspecv1.MediaTypeImageLayerGzip:      &compression.Gzip,
-		imgspecv1.MediaTypeImageLayerZstd:      &compression.Zstd,
-		manifest.DockerV2Schema2LayerMediaType: &compression.Gzip,
+		imgspecv1.MediaTypeImageLayerGzip:         &compression.Gzip,
+		imgspecv1.MediaTypeImageLayerZstd:         &compression.Zstd,
+		manifest.DockerV2Schema2LayerMediaType:    &compression.Gzip,
+		manifest.DockerV2SchemaLayerMediaTypeZstd: &compression.Zstd,
 	}
 )
 

image/internal/image/docker_schema2.go

@@ -233,6 +233,8 @@ func (m *manifestSchema2) convertToManifestOCI1(ctx context.Context, _ *types.Ma
 			layers[idx].MediaType = imgspecv1.MediaTypeImageLayer
 		case manifest.DockerV2Schema2LayerMediaType:
 			layers[idx].MediaType = imgspecv1.MediaTypeImageLayerGzip
+		case manifest.DockerV2SchemaLayerMediaTypeZstd:
+			layers[idx].MediaType = imgspecv1.MediaTypeImageLayerZstd
 		default:
 			return nil, fmt.Errorf("Unknown media type during manifest conversion: %q", m.m.LayersDescriptors[idx].MediaType)
 		}

image/internal/image/oci.go

@@ -288,7 +288,7 @@ func (m *manifestOCI1) convertToManifestSchema2(_ context.Context, options *type
 		case imgspecv1.MediaTypeImageLayerGzip:
 			layers[idx].MediaType = manifest.DockerV2Schema2LayerMediaType
 		case imgspecv1.MediaTypeImageLayerZstd:
-			return nil, fmt.Errorf("Error during manifest conversion: %q: zstd compression is not supported for docker images", layers[idx].MediaType)
+			return nil, fmt.Errorf("Error during manifest conversion: %q: zstd compression is not officially supported for docker images", layers[idx].MediaType)
 		case ociencspec.MediaTypeLayerEnc, ociencspec.MediaTypeLayerGzipEnc, ociencspec.MediaTypeLayerZstdEnc,
 			ociencspec.MediaTypeLayerNonDistributableEnc, ociencspec.MediaTypeLayerNonDistributableGzipEnc, ociencspec.MediaTypeLayerNonDistributableZstdEnc:
 			return nil, fmt.Errorf("during manifest conversion: encrypted layers (%q) are not supported in docker images", layers[idx].MediaType)

image/manifest/manifest.go

@@ -27,6 +27,7 @@ const (
 	// DockerV2SchemaLayerMediaTypeUncompressed is the mediaType used for uncompressed layers.
 	DockerV2SchemaLayerMediaTypeUncompressed = manifest.DockerV2SchemaLayerMediaTypeUncompressed
 	// DockerV2SchemaLayerMediaTypeZstd is the mediaType used for zstd layers.
+	// Warning: This mediaType is not officially supported in https://github.com/distribution/distribution/blob/main/docs/content/spec/manifest-v2-2.md but some images may exhibit it. Support is partial.
 	DockerV2SchemaLayerMediaTypeZstd = manifest.DockerV2SchemaLayerMediaTypeZstd
 	// DockerV2ListMediaType MIME type represents Docker manifest schema 2 list
 	DockerV2ListMediaType = manifest.DockerV2ListMediaType