fix: applying suggestions from @Wwwsylvia

Signed-off-by: Kyle M. Tarplee <kmtarplee@ieee.org>

Author: ktarplee

registry/remote/repository.go

@@ -483,13 +483,12 @@ func (r *Repository) Referrers(ctx context.Context, desc ocispec.Descriptor, art
 
 	// The referrers state is unknown.
 	if err != nil {
-		if !errors.Is(err, errdef.ErrUnsupported) {
-			return err
-		}
-		if errutil.IsErrorCode(err, errcode.ErrorCodeNameUnknown) {
-			// The repository is not found, no fallback.
-			return err
+		if errors.Is(err, errdef.ErrUnsupported) {
+			// Referrers API is not supported, fallback to referrers tag schema.
+			r.SetReferrersCapability(false)
+			return r.referrersByTagSchema(ctx, desc, artifactType, fn)
 		}
+		return err
 		// Fallback to referrers tag schema.
 		r.SetReferrersCapability(false)
 		return r.referrersByTagSchema(ctx, desc, artifactType, fn)
@@ -545,16 +544,19 @@ func (r *Repository) referrersPageByAPI(ctx context.Context, artifactType string
 	switch resp.StatusCode {
 	case http.StatusOK:
 	case http.StatusNotFound:
-		// A 404 returned by Referrers API indicates that Referrers API is not supported.
-		return "", errors.Join(errdef.ErrUnsupported, errutil.ParseErrorResponse(resp))
+		if errResp := errutil.ParseErrorResponse(resp); errutil.IsErrorCode(errResp, errcode.ErrorCodeNameUnknown) {
+			// The repository is not found, Referrers API status is unknown
+			return "", errResp
+		}
+		// Referrers API is not supported.
+		return "", fmt.Errorf("failed to query referrers API: %w", errdef.ErrUnsupported)
 	default:
 		return "", errutil.ParseErrorResponse(resp)
 	}
 
 	// also check the content type
 	if ct := resp.Header.Get("Content-Type"); ct != ocispec.MediaTypeImageIndex {
-		err := errors.Join(errdef.ErrUnsupported, errutil.ParseErrorResponse(resp))
-		return "", fmt.Errorf("unknown content returned (%s), expecting image index: %w", ct, err)
+		return "", fmt.Errorf("unknown content returned (%s), expecting image index: %w", ct, errdef.ErrUnsupported)
 	}
 
 	var index ocispec.Index