download/oci: Set Accept headers (#8720)

The ociTarget.Resolve and ociTarget.Fetch methods introduced in the
containerd migration were not setting Accept headers on manifest
requests. Several registries, including ghcr.io, require a manifest
media type in the Accept header and return 404 Not Found without it.

The previous containerd-based implementation set this header
automatically via the docker resolver. The new oras-based custom target
did not replicate that behaviour.

Signed-off-by: Charlie Egan <charlie_egan@apple.com>

v1/download/oci_download.go

@@ -371,12 +371,23 @@ func (t *ociTarget) url(path string) string {
 	return fmt.Sprintf("%s://%s/v2/%s/%s", scheme, t.registry, t.repo, path)
 }
 
+// manifestMediaTypes lists the OCI and Docker manifest media types that registries
+// such as ghcr.io require in the Accept header to return manifests correctly.
+var manifestMediaTypes = strings.Join([]string{
+	"application/vnd.oci.image.manifest.v1+json",
+	"application/vnd.oci.image.index.v1+json",
+	"application/vnd.docker.distribution.manifest.v2+json",
+	"application/vnd.docker.distribution.manifest.list.v2+json",
+	"*/*",
+}, ", ")
+
 func (t *ociTarget) Resolve(ctx context.Context, reference string) (ocispec.Descriptor, error) {
 	url := t.url("manifests/" + reference)
 	req, err := http.NewRequestWithContext(ctx, http.MethodHead, url, nil)
 	if err != nil {
 		return ocispec.Descriptor{}, err
 	}
+	req.Header.Set("Accept", manifestMediaTypes)
 
 	resp, err := t.client.Do(req)
 	if err != nil {
@@ -408,12 +419,14 @@ func (t *ociTarget) Resolve(ctx context.Context, reference string) (ocispec.Desc
 func (t *ociTarget) Fetch(ctx context.Context, target ocispec.Descriptor) (io.ReadCloser, error) {
 	// Use blobs endpoint for non-manifest content, manifests endpoint for manifests
 	var url string
+	isManifest := false
 	switch target.MediaType {
 	case "application/vnd.oci.image.manifest.v1+json",
 		"application/vnd.oci.image.index.v1+json",
 		"application/vnd.docker.distribution.manifest.v2+json",
 		"application/vnd.docker.distribution.manifest.list.v2+json":
 		url = t.url("manifests/" + target.Digest.String())
+		isManifest = true
 	default:
 		url = t.url("blobs/" + target.Digest.String())
 	}
@@ -422,6 +435,9 @@ func (t *ociTarget) Fetch(ctx context.Context, target ocispec.Descriptor) (io.Re
 	if err != nil {
 		return nil, err
 	}
+	if isManifest {
+		req.Header.Set("Accept", target.MediaType)
+	}
 
 	resp, err := t.client.Do(req)
 	if err != nil {

v1/download/oci_target_test.go

@@ -0,0 +1,71 @@
+//go:build !opa_no_oci
+
+package download
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	digest "github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"oras.land/oras-go/v2/registry/remote/auth"
+)
+
+// TestOCITargetAcceptHeaders verifies that ociTarget sends Accept headers on
+// manifest requests. Registries such as ghcr.io return 404 for HEAD/GET requests
+// that omit the Accept header; this test enforces the same behaviour.
+func TestOCITargetAcceptHeaders(t *testing.T) {
+	const mediaType = "application/vnd.oci.image.manifest.v1+json"
+	digestHex := "sha256:" + strings.Repeat("a", 64)
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// reject any request missing Accept entirely
+		if r.Header.Get("Accept") == "" {
+			w.WriteHeader(http.StatusNotFound)
+			return
+		}
+		// for GET (Fetch), Accept must be the exact descriptor media type
+		if r.Method == http.MethodGet && r.Header.Get("Accept") != mediaType {
+			w.WriteHeader(http.StatusNotAcceptable)
+			return
+		}
+		w.Header().Set("Content-Type", mediaType)
+		w.Header().Set("Docker-Content-Digest", digestHex)
+		w.Header().Set("Content-Length", "100")
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	serverURL, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	target := &ociTarget{
+		client:    &auth.Client{Client: http.DefaultClient, Cache: auth.NewCache()},
+		registry:  serverURL.Host,
+		repo:      "org/repo",
+		plainHTTP: true,
+	}
+
+	// Resolve uses HEAD — any non-empty Accept is sufficient
+	if _, err := target.Resolve(context.Background(), "latest"); err != nil {
+		t.Fatalf("Resolve failed (Accept header likely missing): %v", err)
+	}
+
+	// Fetch uses GET — Accept must match the descriptor's media type exactly
+	desc := ocispec.Descriptor{
+		MediaType: mediaType,
+		Digest:    digest.Digest(digestHex),
+		Size:      100,
+	}
+	rc, err := target.Fetch(context.Background(), desc)
+	if err != nil {
+		t.Fatalf("Fetch failed (Accept header likely missing or wrong): %v", err)
+	}
+	rc.Close()
+}