Add support for gzip-compressed CPIO output

internal/cli/build-cpio.go → internal/cli/build_cpio.go

@@ -15,10 +15,12 @@
 package cli
 
 import (
+	"compress/gzip"
 	"context"
 	"fmt"
 	"os"
 	"runtime"
+	"strings"
 
 	"github.com/spf13/cobra"
 
@@ -109,8 +111,15 @@ func BuildCPIOCmd(ctx context.Context, dest string, opts ...build.Option) error
 	}
 	defer f.Close()
 
-	// TODO(mattmoor): Consider wrapping in a gzip writer if the filename
-	// ends in .gz
+	// Use a gzip-compressed writer when the output filename ends with .gz.
+	if strings.HasSuffix(dest, ".gz") {
+		gzw := gzip.NewWriter(f)
+		if err := cpio.FromLayer(layer, gzw); err != nil {
+			gzw.Close()
+			return err
+		}
+		return gzw.Close()
+	}
 
 	return cpio.FromLayer(layer, f)
 }

internal/cli/build_cpio_test.go

@@ -0,0 +1,62 @@
+// Copyright 2026 Chainguard, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cli_test
+
+import (
+	"compress/gzip"
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"chainguard.dev/apko/internal/cli"
+
+	"chainguard.dev/apko/pkg/build"
+)
+
+func TestBuildCPIOCmd(t *testing.T) {
+	ctx := context.Background()
+
+	t.Run("gz suffix produces gzip-compressed output", func(t *testing.T) {
+		dest := filepath.Join(t.TempDir(), "out.cpio.gz")
+
+		err := cli.BuildCPIOCmd(ctx, dest, build.WithConfig("testdata/apko.yaml", []string{}))
+		require.NoError(t, err)
+
+		f, err := os.Open(dest)
+		require.NoError(t, err)
+		defer f.Close()
+
+		gzr, err := gzip.NewReader(f)
+		require.NoError(t, err, "expected output to be valid gzip")
+		defer gzr.Close()
+	})
+
+	t.Run("non-gz suffix produces plain cpio output", func(t *testing.T) {
+		dest := filepath.Join(t.TempDir(), "out.cpio")
+
+		err := cli.BuildCPIOCmd(ctx, dest, build.WithConfig("testdata/apko.yaml", []string{}))
+		require.NoError(t, err)
+
+		f, err := os.Open(dest)
+		require.NoError(t, err)
+		defer f.Close()
+
+		_, err = gzip.NewReader(f)
+		require.Error(t, err, "expected plain cpio, got gzip")
+	})
+}

pkg/build/layers.go

@@ -299,8 +299,8 @@ func splitLayers(ctx context.Context, fsys apkfs.FullFS, groups []*group, pkgToD
 		// Maintain our "main" stack.
 		if f.header.Typeflag == tar.TypeDir {
 			// Pop off any directories that are not parents of the current file's directory.
-			for i := len(stack) - 1; i >= 0; i-- {
-				if stack[i].path == path.Dir(f.path) {
+			for i, v := range slices.Backward(stack) {
+				if v.path == path.Dir(f.path) {
 					break
 				}