feat: oras discover should show a placeholder of artifactType if it's…

… not presented (oras-project#1336)

Signed-off-by: Xiaoxuan Wang <[email protected]>

cmd/oras/internal/display/metadata/tree/discover.go

@@ -61,6 +61,9 @@ func (h *discoverHandler) OnDiscovered(referrer, subject ocispec.Descriptor) err
 	if !ok {
 		return fmt.Errorf("unexpected subject descriptor: %v", subject)
 	}
+	if referrer.ArtifactType == "" {
+		referrer.ArtifactType = "<unknown>"
+	}
 	referrerNode := node.AddPath(referrer.ArtifactType, referrer.Digest)
 	if h.verbose {
 		for k, v := range referrer.Annotations {

test/e2e/suite/command/discover.go

@@ -172,6 +172,12 @@ var _ = Describe("1.1 registry users:", func() {
 				MatchKeyWords(append(discoverKeyWords(true, referrers...), RegistryRef(ZOTHost, ArtifactRepo, foobar.Digest))...).
 				Exec()
 		})
+
+		It("should display <unknown> if a referrer has an empty artifact type", func() {
+			ORAS("discover", RegistryRef(ZOTHost, ArtifactRepo, "multi"), "--format", format).
+				MatchKeyWords("<unknown>").
+				Exec()
+		})
 	})
 	When("running discover command with table output", func() {
 		format := "table"