libimage/layer_tree: if parent is empty and manifest list ignore

Layer tree expectes to form a relation between child and parent
instances, however it expects an instance from manifest list which is
empty, following expectation is not possible and will always resuilt in
error.

Closes: containers/podman#19860

[NO NEW TESTS NEEDED]
Image without layer cant be built in libimage, and `podman save`
automatically malforms such image so no such external image can be
loaded.

Signed-off-by: Aditya R <arajan@redhat.com>

libimage/layer_tree.go

@@ -289,6 +289,15 @@ func (t *layerTree) parent(ctx context.Context, child *Image) (*Image, error) {
 			if childID == empty.ID() {
 				continue
 			}
+			isManifest, err := empty.IsManifestList(ctx)
+			if err != nil {
+				return nil, err
+			}
+			if isManifest {
+				// If this is a manifest list and is already
+				// marked as empty then no instance can be
+				continue
+			}
 			emptyOCI, err := t.toOCI(ctx, empty)
 			if err != nil {
 				if ErrorIsImageUnknown(err) {

libimage/list_test.go

@@ -0,0 +1,54 @@
+package libimage
+
+import (
+	"context"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestListWithEmptyLayerImage(t *testing.T) {
+	runtime, cleanup := testNewRuntime(t)
+	defer cleanup()
+	ctx := context.Background()
+	loadOptions := &LoadOptions{}
+	loadOptions.Writer = os.Stdout
+
+	list, err := runtime.CreateManifestList("m")
+	require.NoError(t, err)
+	require.NotNil(t, list)
+
+	for _, test := range []struct {
+		input       string
+		expectError bool
+		numImages   int
+		names       []string
+	}{
+		// OCI ARCHIVE
+		{"testdata/empty_image.tar", false, 1, []string{"localhost/i:latest"}},
+	} {
+		loadedImages, err := runtime.Load(ctx, test.input, loadOptions)
+		if test.expectError {
+			require.Error(t, err, test.input)
+			continue
+		}
+		require.NoError(t, err, test.input)
+		require.Equal(t, test.names, loadedImages, test.input)
+
+		// Make sure that all returned names exist as images in the
+		// local containers storage.
+		ids := []string{} // later used for image removal
+		names := [][]string{}
+		for _, name := range loadedImages {
+			image, resolvedName, err := runtime.LookupImage(name, nil)
+			require.NoError(t, err, test.input)
+			require.Equal(t, name, resolvedName, test.input)
+			ids = append(ids, image.ID())
+			names = append(names, image.Names())
+		}
+	}
+	listedImages, err := runtime.ListImages(ctx, nil, &ListImagesOptions{})
+	require.NoError(t, err)
+	require.Len(t, listedImages, 2, "expected 2 images")
+}