Support referrers to a missing manifest

Signed-off-by: Brandon Mitchell <git@bmitch.net>

conformance/03_discovery_test.go

@@ -237,6 +237,18 @@ var test03ContentDiscovery = func() {
 					BeNumerically(">=", 200),
 					BeNumerically("<", 300)))
 				Expect(resp.Header().Get("OCI-Subject")).To(Equal(manifests[4].Digest))
+
+				// Populate registry with test references manifest to a non-existent subject
+				req = client.NewRequest(reggie.PUT, "/v2/<name>/manifests/<reference>",
+					reggie.WithReference(refsManifestCLayerArtifactDigest)).
+					SetHeader("Content-Type", "application/vnd.oci.image.manifest.v1+json").
+					SetBody(refsManifestCLayerArtifactContent)
+				resp, err = client.Do(req)
+				Expect(err).To(BeNil())
+				Expect(resp.StatusCode()).To(SatisfyAll(
+					BeNumerically(">=", 200),
+					BeNumerically("<", 300)))
+				Expect(resp.Header().Get("OCI-Subject")).To(Equal(manifests[3].Digest))
 			})
 		})
 
@@ -345,6 +357,22 @@ var test03ContentDiscovery = func() {
 					Warn("filtering by artifact-type is not implemented")
 				}
 			})
+
+			g.Specify("GET request to missing manifest should yield 200", func() {
+				SkipIfDisabled(contentDiscovery)
+				req := client.NewRequest(reggie.GET, "/v2/<name>/referrers/<digest>",
+					reggie.WithDigest(manifests[3].Digest))
+				resp, err := client.Do(req)
+				Expect(err).To(BeNil())
+				Expect(resp.StatusCode()).To(Equal(http.StatusOK))
+				Expect(resp.Header().Get("Content-Type")).To(Equal("application/vnd.oci.image.index.v1+json"))
+
+				var index index
+				err = json.Unmarshal(resp.Body(), &index)
+				Expect(err).To(BeNil())
+				Expect(len(index.Manifests)).To(Equal(1))
+				Expect(index.Manifests[0].Digest.String()).To(Equal(refsManifestCLayerArtifactDigest))
+			})
 		})
 
 		g.Context("Teardown", func() {
@@ -361,6 +389,7 @@ var test03ContentDiscovery = func() {
 						testTagName,
 						refsManifestBConfigArtifactDigest,
 						refsManifestBLayerArtifactDigest,
+						refsManifestCLayerArtifactDigest,
 					}
 					for _, ref := range references {
 						req := client.NewRequest(reggie.DELETE, "/v2/<name>/manifests/<digest>", reggie.WithDigest(ref))
@@ -421,6 +450,7 @@ var test03ContentDiscovery = func() {
 						testTagName,
 						refsManifestBConfigArtifactDigest,
 						refsManifestBLayerArtifactDigest,
+						refsManifestCLayerArtifactDigest,
 					}
 					for _, ref := range references {
 						req := client.NewRequest(reggie.DELETE, "/v2/<name>/manifests/<digest>", reggie.WithDigest(ref))

conformance/setup.go

@@ -147,6 +147,8 @@ var (
 	refsManifestBConfigArtifactDigest  string
 	refsManifestBLayerArtifactContent  []byte
 	refsManifestBLayerArtifactDigest   string
+	refsManifestCLayerArtifactContent  []byte
+	refsManifestCLayerArtifactDigest   string
 	refsIndexArtifactContent           []byte
 	refsIndexArtifactDigest            string
 	reportJUnitFilename                string
@@ -440,6 +442,33 @@ func init() {
 
 	refsManifestBLayerArtifactDigest = godigest.FromBytes(refsManifestBLayerArtifactContent).String()
 
+	// ManifestCLayerArtifact is the same as B but based on a subject that has not been pushed
+	refsManifestCLayerArtifact := manifest{
+		SchemaVersion: 2,
+		MediaType:     "application/vnd.oci.image.manifest.v1+json",
+		ArtifactType:  testRefArtifactTypeB,
+		Config:        emptyJSONDescriptor,
+		Subject: &descriptor{
+			MediaType: "application/vnd.oci.image.manifest.v1+json",
+			Size:      int64(len(manifests[3].Content)),
+			Digest:    godigest.FromBytes(manifests[3].Content),
+		},
+		Layers: []descriptor{
+			{
+				MediaType: testRefArtifactTypeB,
+				Size:      int64(len(testRefBlobB)),
+				Digest:    godigest.FromBytes(testRefBlobB),
+			},
+		},
+	}
+
+	refsManifestCLayerArtifactContent, err = json.MarshalIndent(&refsManifestCLayerArtifact, "", "\t")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	refsManifestCLayerArtifactDigest = godigest.FromBytes(refsManifestCLayerArtifactContent).String()
+
 	testRefArtifactTypeIndex = "application/vnd.food.stand"
 	refsIndexArtifact := index{
 		SchemaVersion: 2,

spec.md

@@ -566,7 +566,6 @@ The descriptors MUST include an `artifactType` field that is set to the value of
 If the `artifactType` is empty or missing in the image manifest, the value of `artifactType` MUST be set to the config descriptor `mediaType` value.
 The descriptors MUST include annotations from the image manifest.
 If a query results in no matching referrers, an empty manifest list MUST be returned.
-If a manifest with the digest `<digest>` does not exist, a registry MAY return an empty manifest list.
 After a manifest with the digest `<digest>` is pushed, the registry MUST include previously pushed entries in the referrers list.
 
 ```json