Detect zstd in crane append

We assumed everything was gzip, which is not always true!

Signed-off-by: Jon Johnson <[email protected]>

pkg/crane/append.go

@@ -18,7 +18,9 @@ import (
 	"fmt"
 	"os"
 
+	comp "github.com/google/go-containerregistry/internal/compression"
 	"github.com/google/go-containerregistry/internal/windows"
+	"github.com/google/go-containerregistry/pkg/compression"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/mutate"
 	"github.com/google/go-containerregistry/pkg/v1/stream"
@@ -50,13 +52,11 @@ func Append(base v1.Image, paths ...string) (v1.Image, error) {
 	}
 
 	baseMediaType, err := base.MediaType()
-
 	if err != nil {
 		return nil, fmt.Errorf("getting base image media type: %w", err)
 	}
 
 	layerType := types.DockerLayer
-
 	if baseMediaType == types.OCIManifestSchema1 {
 		layerType = types.OCILayer
 	}
@@ -90,6 +90,21 @@ func getLayer(path string, layerType types.MediaType) (v1.Layer, error) {
 		return stream.NewLayer(f, stream.WithMediaType(layerType)), nil
 	}
 
+	// This is dumb but the tarball package assumes things about mediaTypes that aren't true
+	// and doesn't have enough context to know what the right default is.
+	f, err = os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	z, _, err := comp.PeekCompression(f)
+	if err != nil {
+		return nil, err
+	}
+	if z == compression.ZStd {
+		layerType = types.OCILayerZStd
+	}
+
 	return tarball.LayerFromFile(path, tarball.WithMediaType(layerType))
 }
 

pkg/crane/append_test.go

@@ -71,3 +71,28 @@ func TestAppendWithDockerBaseImage(t *testing.T) {
 		t.Errorf("MediaType(): want %q, got %q", want, got)
 	}
 }
+
+func TestAppendWithZstd(t *testing.T) {
+	base := mutate.MediaType(empty.Image, types.OCIManifestSchema1)
+	img, err := crane.Append(base, "testdata/content.tar.zst")
+
+	if err != nil {
+		t.Fatalf("crane.Append(): %v", err)
+	}
+
+	layers, err := img.Layers()
+
+	if err != nil {
+		t.Fatalf("img.Layers(): %v", err)
+	}
+
+	mediaType, err := layers[0].MediaType()
+
+	if err != nil {
+		t.Fatalf("layers[0].MediaType(): %v", err)
+	}
+
+	if got, want := mediaType, types.OCILayerZStd; got != want {
+		t.Errorf("MediaType(): want %q, got %q", want, got)
+	}
+}