Reject a tar manifest whose descriptor carries no refs

pkg/imgpkg/imagedesc/image_ref_descriptors.go

@@ -51,9 +51,49 @@ func NewImageRefDescriptorsFromBytes(data []byte) (*ImageRefDescriptors, error)
 		return nil, err
 	}
 
+	for _, desc := range descs {
+		switch {
+		case desc.Image != nil:
+			if err := validateImageRefs(*desc.Image); err != nil {
+				return nil, err
+			}
+		case desc.ImageIndex != nil:
+			if err := validateImageIndexRefs(*desc.ImageIndex); err != nil {
+				return nil, err
+			}
+		}
+	}
+
 	return &ImageRefDescriptors{descs: descs}, nil
 }
 
+// validateImageRefs checks that a descriptor carries the ref that Ref() reads.
+// Every descriptor this package writes has exactly one, but nothing stops a
+// hand-written or truncated manifest from arriving without any.
+func validateImageRefs(desc ImageDescriptor) error {
+	if len(desc.Refs) == 0 {
+		return fmt.Errorf("Expected image descriptor %s to have at least one ref", desc.Manifest.Digest)
+	}
+	return nil
+}
+
+func validateImageIndexRefs(desc ImageIndexDescriptor) error {
+	if len(desc.Refs) == 0 {
+		return fmt.Errorf("Expected image index descriptor %s to have at least one ref", desc.Digest)
+	}
+	for _, img := range desc.Images {
+		if err := validateImageRefs(img); err != nil {
+			return err
+		}
+	}
+	for _, idx := range desc.Indexes {
+		if err := validateImageIndexRefs(idx); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func NewImageRefDescriptors(refs []Metadata, registry Registry) (*ImageRefDescriptors, error) {
 	registry = errRegistry{registry}
 

pkg/imgpkg/imagedesc/image_ref_descriptors_test.go

@@ -0,0 +1,49 @@
+// Copyright 2024 The Carvel Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package imagedesc_test
+
+import (
+	"strings"
+	"testing"
+
+	"carvel.dev/imgpkg/pkg/imgpkg/imagedesc"
+)
+
+func TestNewImageRefDescriptorsFromBytes(t *testing.T) {
+	t.Run("rejects an image descriptor with no refs", func(t *testing.T) {
+		manifest := `[{"Image":{"Refs":[],"Manifest":{"Digest":"sha256:aaaa"}}}]`
+
+		_, err := imagedesc.NewImageRefDescriptorsFromBytes([]byte(manifest))
+		if err == nil {
+			t.Fatal("expected an error, got none")
+		}
+		if !strings.Contains(err.Error(), "at least one ref") {
+			t.Errorf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("rejects an image nested in an index with no refs", func(t *testing.T) {
+		manifest := `[{"ImageIndex":{"Refs":["registry.example.com/repo@sha256:bbbb"],"Digest":"sha256:bbbb","Images":[{"Refs":[],"Manifest":{"Digest":"sha256:aaaa"}}]}}]`
+
+		_, err := imagedesc.NewImageRefDescriptorsFromBytes([]byte(manifest))
+		if err == nil {
+			t.Fatal("expected an error, got none")
+		}
+		if !strings.Contains(err.Error(), "at least one ref") {
+			t.Errorf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("accepts a descriptor with a ref", func(t *testing.T) {
+		manifest := `[{"Image":{"Refs":["registry.example.com/repo@sha256:aaaa"],"Manifest":{"Digest":"sha256:aaaa"}}}]`
+
+		ids, err := imagedesc.NewImageRefDescriptorsFromBytes([]byte(manifest))
+		if err != nil {
+			t.Fatalf("got an error: %v", err)
+		}
+		if got := len(ids.Descriptors()); got != 1 {
+			t.Errorf("expected 1 descriptor, got %d", got)
+		}
+	})
+}