Fall back to config media type for old-style artifact types

cmd/oras/root/manifest/index/create.go

@@ -245,6 +245,18 @@ func enrichDescriptor(ctx context.Context, target oras.ReadOnlyTarget, desc ocis
 			return ocispec.Descriptor{}, err
 		}
 		desc.ArtifactType = manifest.ArtifactType
+		if desc.ArtifactType == "" {
+			// Old-style artifacts predate the manifest artifactType field and
+			// convey their type through config.mediaType instead. Per the
+			// image-spec descriptor guidance, fall back to config.mediaType so
+			// the enriched descriptor still advertises an artifactType.
+			switch manifest.Config.MediaType {
+			case ocispec.MediaTypeImageConfig, ocispec.MediaTypeEmptyJSON, docker.MediaTypeConfig:
+				// standard image / empty config: not an artifact type, leave empty
+			default:
+				desc.ArtifactType = manifest.Config.MediaType
+			}
+		}
 	} else if descriptor.IsIndex(desc) {
 		var index ocispec.Index
 		if err := json.Unmarshal(manifestBytes, &index); err != nil {

cmd/oras/root/manifest/index/create_test.go

@@ -292,6 +292,102 @@ func Test_enrichDescriptor(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name:   "old-style artifact, artifactType falls back to config mediaType",
+			target: NewTestReadOnlyTarget(`intentionally not valid JSON`),
+			manifestBytes: []byte(`
+				{
+					"schemaVersion": 2,
+					"mediaType": "application/vnd.oci.image.manifest.v1+json",
+					"config": {
+						"mediaType": "application/vnd.example.config",
+						"digest": "sha256:dc889043956f34871cc04ae96e03efc29dfe2f582c26195a72dd4827f4dd830d",
+						"size": 28
+					},
+					"layers": []
+				}
+			`),
+			manifestMediaType: "application/vnd.oci.image.manifest.v1+json",
+			checkDesc: func(t *testing.T, gotDesc, _ ocispec.Descriptor) {
+				t.Helper()
+				if got, want := gotDesc.ArtifactType, "application/vnd.example.config"; got != want {
+					t.Errorf("ArtifactType = %s, want %s", got, want)
+				}
+			},
+			wantErr: false,
+		},
+		{
+			name:   "empty artifactType with OCI image config stays empty",
+			target: NewTestReadOnlyTarget(`intentionally not valid JSON`),
+			manifestBytes: []byte(`
+				{
+					"schemaVersion": 2,
+					"mediaType": "application/vnd.oci.image.manifest.v1+json",
+					"config": {
+						"mediaType": "application/vnd.oci.image.config.v1+json",
+						"digest": "sha256:dc889043956f34871cc04ae96e03efc29dfe2f582c26195a72dd4827f4dd830d",
+						"size": 28
+					},
+					"layers": []
+				}
+			`),
+			manifestMediaType: "application/vnd.oci.image.manifest.v1+json",
+			checkDesc: func(t *testing.T, gotDesc, _ ocispec.Descriptor) {
+				t.Helper()
+				if got := gotDesc.ArtifactType; got != "" {
+					t.Errorf("ArtifactType = %q, want empty", got)
+				}
+			},
+			wantErr: false,
+		},
+		{
+			name:   "empty artifactType with empty JSON config stays empty",
+			target: NewTestReadOnlyTarget(`intentionally not valid JSON`),
+			manifestBytes: []byte(`
+				{
+					"schemaVersion": 2,
+					"mediaType": "application/vnd.oci.image.manifest.v1+json",
+					"config": {
+						"mediaType": "application/vnd.oci.empty.v1+json",
+						"digest": "sha256:dc889043956f34871cc04ae96e03efc29dfe2f582c26195a72dd4827f4dd830d",
+						"size": 28
+					},
+					"layers": []
+				}
+			`),
+			manifestMediaType: "application/vnd.oci.image.manifest.v1+json",
+			checkDesc: func(t *testing.T, gotDesc, _ ocispec.Descriptor) {
+				t.Helper()
+				if got := gotDesc.ArtifactType; got != "" {
+					t.Errorf("ArtifactType = %q, want empty", got)
+				}
+			},
+			wantErr: false,
+		},
+		{
+			name:   "empty artifactType with docker image config stays empty",
+			target: NewTestReadOnlyTarget(`intentionally not valid JSON`),
+			manifestBytes: []byte(`
+				{
+					"schemaVersion": 2,
+					"mediaType": "application/vnd.oci.image.manifest.v1+json",
+					"config": {
+						"mediaType": "application/vnd.docker.container.image.v1+json",
+						"digest": "sha256:dc889043956f34871cc04ae96e03efc29dfe2f582c26195a72dd4827f4dd830d",
+						"size": 28
+					},
+					"layers": []
+				}
+			`),
+			manifestMediaType: "application/vnd.oci.image.manifest.v1+json",
+			checkDesc: func(t *testing.T, gotDesc, _ ocispec.Descriptor) {
+				t.Helper()
+				if got := gotDesc.ArtifactType; got != "" {
+					t.Errorf("ArtifactType = %q, want empty", got)
+				}
+			},
+			wantErr: false,
+		},
 		{
 			name:              "child of unrecognized type",
 			target:            NewTestReadOnlyTarget("(unused)"),

test/e2e/internal/testdata/nonjson_config/const.go

@@ -21,8 +21,9 @@ import (
 
 var (
 	Descriptor = ocispec.Descriptor{
-		MediaType: "application/vnd.oci.image.manifest.v1+json",
-		Digest:    "sha256:9d16f5505246424aed7116cb21216704ba8c919997d0f1f37e154c11d509e1d2",
-		Size:      529,
+		MediaType:    "application/vnd.oci.image.manifest.v1+json",
+		Digest:       "sha256:9d16f5505246424aed7116cb21216704ba8c919997d0f1f37e154c11d509e1d2",
+		Size:         529,
+		ArtifactType: "application/vnd.unknown.config.v1+json",
 	}
 )