fix: allow pre-v3.7.0 HELM Charts in HELM OCI Artifact downloader (#1476) (#1479)

<!-- markdownlint-disable MD041 -->
#### What this PR does / why we need it

The release of Helm v3.7.0 included the implementation of HIP 6 for OCI
support. The chart layer media type was switched from
application/tar+gzip to
application/vnd.cncf.helm.chart.content.v1.tar+gzip.

To support HELM Charts created before this version, we should maintain
compatibility with a legacy layer detection.

This implements this detection to make sure we don't have breaking
changes after our recent adjustment to media type based checking.

See

https://helm.sh/docs/topics/registries/#oci-feature-deprecation-and-behavior-changes-with-v370
for context

#### Which issue(s) this PR fixes
<!--
Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`. -->
fix #1469

relates to #1470 and
needs to be backported the same

(cherry picked from commit 729a19f)

Author: jakobmoellerdev

api/ocm/extensions/download/handlers/helm/download_test.go

@@ -33,6 +33,7 @@ const (
 	ArtifactType       = "NotHelmChart"
 	SpecialOCIResource = "specialhelm"
 	UnusualOCIResource = "unusualhelm"
+	LegacyOCIResource  = "legacyhelm"
 )
 
 var _ = Describe("upload", func() {
@@ -53,6 +54,9 @@ var _ = Describe("upload", func() {
 					env.Resource(UnusualOCIResource, Version, resourcetypes.HELM_CHART, v1.LocalRelation, func() {
 						env.BlobFromFile(artifactset.MediaType(artdesc.MediaTypeImageManifest), filepath.Join("/testdata/unusual-ordered-helm-chart.tgz"))
 					})
+					env.Resource(LegacyOCIResource, Version, resourcetypes.HELM_CHART, v1.LocalRelation, func() {
+						env.BlobFromFile(artifactset.MediaType(artdesc.MediaTypeImageManifest), filepath.Join("/testdata/legacy-pre-hip-helm-chart.tgz"))
+					})
 				})
 			})
 		})
@@ -88,4 +92,14 @@ var _ = Describe("upload", func() {
 		MustBeSuccessful(tarutils.ExtractArchiveToFs(env.FileSystem(), path, env.FileSystem()))
 		Expect(Must(vfs.FileExists(env.FileSystem(), "/postgresql/Chart.yaml"))).To(BeTrue())
 	})
+
+	It("successfully download artifacts with helm chart content with legacy content type", func() {
+		MustBeSuccessful(download.RegisterHandlerByName(env, helm.PATH, nil, download.ForArtifactType(ArtifactType)))
+
+		repo := Must(ctf.Open(env.OCMContext(), accessobj.ACC_READONLY, CTFPath, 0o777, env))
+		cv := Must(repo.LookupComponentVersion(Component, Version))
+		path := Must(download.DownloadResource(env.OCMContext(), Must(cv.SelectResources(selectors.Identity(v1.Identity{"name": LegacyOCIResource})))[0], "/resource", download.WithFileSystem(env.FileSystem())))
+		MustBeSuccessful(tarutils.ExtractArchiveToFs(env.FileSystem(), path, env.FileSystem()))
+		Expect(Must(vfs.FileExists(env.FileSystem(), "/ingress-nginx/Chart.yaml"))).To(BeTrue())
+	})
 })

api/ocm/extensions/download/handlers/helm/handler.go

@@ -129,7 +129,12 @@ func download(p common.Printer, art oci.ArtifactAccess, path string, fs vfs.File
 
 	chartDesc, err := findLayer(desc.Layers, helmregistry.ChartLayerMediaType)
 	if err != nil {
-		return "", "", fmt.Errorf("no valid chart layer found: %w", err)
+		// Fallback to legacy chart layer media type if no chart layer is found in the normal order.
+		// See https://helm.sh/docs/topics/registries/#oci-feature-deprecation-and-behavior-changes-with-v370
+		var fallbackErr error
+		if chartDesc, fallbackErr = findLayer(desc.Layers, helmregistry.LegacyChartLayerMediaType); fallbackErr != nil {
+			return "", "", errors.Join(err, fallbackErr)
+		}
 	}
 
 	chartBlob, err := m.GetBlob(chartDesc.Digest)