Define image manifest artifactType and guidance

Signed-off-by: Brandon Mitchell <[email protected]>

manifest.md

@@ -26,6 +26,12 @@ Unlike the [image index](image-index.md), which contains information about a set
   When used, this field MUST contain the media type `application/vnd.oci.image.manifest.v1+json`.
   This field usage differs from the [descriptor](descriptor.md#properties) use of `mediaType`.
 
+- **`artifactType`** *string*
+
+  This OPTIONAL property contains the type of an artifact when the manifest is used for an artifact.
+  This MUST be set when `config.mediaType` is set to the [scratch value](#example-of-a-scratch-config-or-layer-descriptor).
+  If defined, the value MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2], and MAY be registered with [IANA][iana].
+
 - **`config`** *[descriptor](descriptor.md)*
 
     This REQUIRED property references a configuration object for a container, by digest.
@@ -42,6 +48,8 @@ Unlike the [image index](image-index.md), which contains information about a set
 
         If the manifest uses a different media type than the above, it MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2], and MAY be registered with [IANA][iana].
 
+        See [Guidelines for Artifact Usage](#guidelines-for-artifact-usage) for other uses of the `config.mediaType` field.
+
     To set an effectively NULL or SCRATCH config and maintain portability the following is considered GUIDANCE.
     While an empty blob (`size` of 0) may be preferable, practice has shown that not to be ubiquitiously supported.
     Instead, the blob payload can be the most minimal content that is still valid JSON object: `{}` (`size` of 2).
@@ -78,6 +86,8 @@ Unlike the [image index](image-index.md), which contains information about a set
 
         If the manifest uses a different media type than the above, it MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2], and MAY be registered with [IANA][iana].
 
+        See [Guidelines for Artifact Usage](#guidelines-for-artifact-usage) for other uses of the `layers`.
+
 - **`subject`** *[descriptor](descriptor.md)*
 
     This OPTIONAL property specifies a [descriptor](descriptor.md) of another manifest.
@@ -133,16 +143,94 @@ Unlike the [image index](image-index.md), which contains information about a set
 
 ## Example of a SCRATCH config or layer descriptor
 
-Notice that the `mediaType` is subject to the usage or context, while the digest is specifically defined as `ScratchDigestSHA256`
+Notice that the `mediaType` is subject to the usage or context, while the digest is specifically defined as `ScratchDigestSHA256`.
+When the `ScratchDigestSHA256` is used, the media type SHOULD be set to `application/vnd.oci.scratch.v1+json` to differentiate the descriptor from one pointing to content.
 
 ```json,title=SCRATCH%20config&mediatype=application/vnd.oci.descriptor.v1%2Bjson
 {
-  "mediaType": "application/vnd.oci.example+json",
+  "mediaType": "application/vnd.oci.scratch.v1+json",
   "size": 2,
   "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
 }
 ```
 
+## Guidelines for Artifact Usage
+
+Content other than OCI container images MAY be packaged using the image manifest.
+When this is done, the `config.mediaType` value MUST be set to a value specific to the artifact type or the [scratch value](#example-of-a-scratch-config-or-layer-descriptor).
+If the `config.mediaType` is set to the scratch value, the `artifactType` must be defined.
+If the artifact does not need layers, a single layer SHOULD be included with a non-zero size.
+The suggested content for an unused layer is the [SCRATCH](#example-of-a-scratch-config-or-layer-descriptor) descriptor.
+
+Here is an example manifest for a typical artifact:
+
+```json,title=Artifact%20with%20config&mediatype=application/vnd.oci.image.manifest.v1%2Bjson
+{
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.oci.image.manifest.v1+json",
+  "config": {
+    "mediaType": "application/vnd.example.config.v1+json",
+    "digest": "sha256:5891b5b522d5df086d0ff0b110fbd9d21bb4fc7163af34d08286a2e846f6be03",
+    "size": 123
+  },
+  "layers": [
+    {
+      "mediaType": "application/vnd.example.data.v1.tar+gzip",
+      "digest": "sha256:e258d248fda94c63753607f7c4494ee0fcbe92f1a76bfdac795c9d84101eb317",
+      "size": 1234
+    }
+  ]
+}
+```
+
+Here is an example manifest for a simple artifact without content in the config, using the scratch descriptor:
+
+```json,title=Artifact%20without%20config&mediatype=application/vnd.oci.image.manifest.v1%2Bjson
+{
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.oci.image.manifest.v1+json",
+  "artifactType": "application/vnd.example+type",
+  "config": {
+    "mediaType": "application/vnd.oci.scratch.v1+json",
+    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
+    "size": 2
+  },
+  "layers": [
+    {
+      "mediaType": "application/vnd.example+type",
+      "digest": "sha256:e258d248fda94c63753607f7c4494ee0fcbe92f1a76bfdac795c9d84101eb317",
+      "size": 1234
+    }
+  ]
+}
+```
+
+Here is an example manifest for an artifact with only annotations set, and the content of both blobs set to the scratch descriptor:
+
+```json,title=Minimal%20artifact&mediatype=application/vnd.oci.image.manifest.v1%2Bjson
+{
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.oci.image.manifest.v1+json",
+  "artifactType": "application/vnd.example+type",
+  "config": {
+    "mediaType": "application/vnd.oci.scratch.v1+json",
+    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
+    "size": 2
+  },
+  "layers": [
+    {
+      "mediaType": "application/vnd.oci.scratch.v1+json",
+      "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
+      "size": 2
+    }
+  ],
+  "annotations": {
+    "oci.opencontainers.image.created": "2023-01-02T03:04:05Z",
+    "com.example.data": "payload"
+  }
+}
+```
+
 [iana]:         https://www.iana.org/assignments/media-types/media-types.xhtml
 [rfc6838]:      https://tools.ietf.org/html/rfc6838
 [rfc6838-s4.2]: https://tools.ietf.org/html/rfc6838#section-4.2

media-types.md

@@ -10,6 +10,7 @@ The following media types identify the formats described here and their referenc
 - `application/vnd.oci.image.layer.v1.tar`: ["Layer", as a tar archive](layer.md)
 - `application/vnd.oci.image.layer.v1.tar+gzip`: ["Layer", as a tar archive](layer.md#gzip-media-types) compressed with [gzip][rfc1952]
 - `application/vnd.oci.image.layer.v1.tar+zstd`: ["Layer", as a tar archive](layer.md#zstd-media-types) compressed with [zstd][rfc8478]
+- `application/vnd.oci.scratch.v1+json`: [Scratch blob](manifest.md#example-of-a-scratch-config-or-layer-descriptor)
 - `application/vnd.oci.artifact.manifest.v1+json`: [Artifact manifest](artifact.md)
 
 The following media types identify a ["Layer" with distribution restrictions](layer.md#non-distributable-layers), but are **deprecated** and not recommended for future use:

schema/image-manifest-schema.json

@@ -15,6 +15,10 @@
       "description": "the mediatype of the referenced object",
       "$ref": "defs-descriptor.json#/definitions/mediaType"
     },
+    "artifactType": {
+      "description": "the artifact mediatype of the referenced object",
+      "$ref": "defs-descriptor.json#/definitions/mediaType"
+    },
     "config": {
       "$ref": "content-descriptor.json"
     },

schema/manifest_test.go

@@ -289,6 +289,53 @@ func TestManifest(t *testing.T) {
 `,
 			fail: true,
 		},
+
+		// valid manifest for an artifact with a dedicated config
+		{
+			manifest: `
+{
+  "schemaVersion": 2,
+  "mediaType" : "application/vnd.oci.image.manifest.v1+json",
+  "config": {
+    "mediaType": "application/vnd.example.config+json",
+    "size": 1470,
+    "digest": "sha256:c86f7763873b6c0aae22d963bab59b4f5debbed6685761b5951584f6efb0633b"
+  },
+  "layers": [
+    {
+      "mediaType": "application/vnd.example.data+type",
+      "size": 675598,
+      "digest": "sha256:9d3dd9504c685a304985025df4ed0283e47ac9ffa9bd0326fddf4d59513f0827"
+    }
+  ]
+}
+`,
+			fail: false,
+		},
+
+		// valid manifest for an artifact using the scratch config and artifactType
+		{
+			manifest: `
+{
+  "schemaVersion": 2,
+  "mediaType" : "application/vnd.oci.image.manifest.v1+json",
+  "artifactType": "application/vnd.example+type",
+  "config": {
+    "mediaType": "application/vnd.oci.scratch.v1+json",
+    "size": 2,
+    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
+  },
+  "layers": [
+    {
+      "mediaType": "application/vnd.example+type",
+      "size": 675598,
+      "digest": "sha256:9d3dd9504c685a304985025df4ed0283e47ac9ffa9bd0326fddf4d59513f0827"
+    }
+  ]
+}
+`,
+			fail: false,
+		},
 	} {
 		r := strings.NewReader(tt.manifest)
 		err := schema.ValidatorMediaTypeManifest.Validate(r)

schema/validator.go

@@ -92,7 +92,7 @@ func (v Validator) Validate(src io.Reader) error {
 
 type unimplemented string
 
-func (v unimplemented) Validate(src io.Reader) error {
+func (v unimplemented) Validate(_ io.Reader) error {
 	return fmt.Errorf("%s: unimplemented", v)
 }
 

specs-go/v1/manifest.go

@@ -23,6 +23,9 @@ type Manifest struct {
 	// MediaType specifies the type of this document data structure e.g. `application/vnd.oci.image.manifest.v1+json`
 	MediaType string `json:"mediaType,omitempty"`
 
+	// ArtifactType specifies the IANA media type of artifact when the manifest is used for an artifact.
+	ArtifactType string `json:"artifactType,omitempty"`
+
 	// Config references a configuration object for a container, by digest.
 	// The referenced configuration object is a JSON blob that the runtime uses to set up the container.
 	Config Descriptor `json:"config"`

specs-go/v1/mediatype.go

@@ -70,6 +70,9 @@ const (
 	// MediaTypeImageConfig specifies the media type for the image configuration.
 	MediaTypeImageConfig = "application/vnd.oci.image.config.v1+json"
 
+	// MediaTypeScratch specifies the media type for an unused blob containing the value `{}`
+	MediaTypeScratch = "application/vnd.oci.scratch.v1+json"
+
 	// MediaTypeArtifactManifest specifies the media type for a content descriptor.
 	MediaTypeArtifactManifest = "application/vnd.oci.artifact.manifest.v1+json"
 )