feat(package): support SOURCE_DATE_EPOCH for reproducible chart archives

Read SOURCE_DATE_EPOCH in the CLI layer (pkg/cmd/package.go) and pass it
to the action layer as Package.SourceDateEpoch *time.Time. Before calling
Save(), stampModTimes() overwrites every tar-entry ModTime in the chart
(including templates, files, raw values, schema and sub-charts) with the
requested epoch.

writeToTar and Save are not modified: the caller now supplies the epoch as
the modTime argument, which is how mattfarina's review (#32060) asked for
this to be structured.

Closes #31439

Signed-off-by: Ilya Kiselev <kis-ilya-a@yandex.ru>

pkg/action/package.go

@@ -23,6 +23,7 @@ import (
 	"os"
 	"path/filepath"
 	"syscall"
+	"time"
 
 	"github.com/Masterminds/semver/v3"
 	"golang.org/x/term"
@@ -49,6 +50,12 @@ type Package struct {
 	Destination      string
 	DependencyUpdate bool
 
+	// SourceDateEpoch, when non-nil, overrides all tar entry modification times
+	// in the produced chart archive. Set by callers that want reproducible builds.
+	// The environment variable SOURCE_DATE_EPOCH is the conventional way to supply
+	// this value from the CLI; reading that variable is the CLI's responsibility.
+	SourceDateEpoch *time.Time
+
 	RepositoryConfig      string
 	RepositoryCache       string
 	PlainHTTP             bool
@@ -121,6 +128,10 @@ func (p *Package) Run(path string, _ map[string]any) (string, error) {
 		dest = p.Destination
 	}
 
+	if p.SourceDateEpoch != nil {
+		stampModTimes(ch, *p.SourceDateEpoch)
+	}
+
 	name, err := chartutil.Save(ch, dest)
 	if err != nil {
 		return "", fmt.Errorf("failed to save: %w", err)
@@ -254,3 +265,22 @@ func openPassphraseFile(passphraseFile string, stdin *os.File) (*os.File, error)
 	}
 	return os.Open(passphraseFile)
 }
+
+// stampModTimes recursively sets all file modification times in a chart to t.
+// This is used to produce reproducible archives when SourceDateEpoch is set.
+func stampModTimes(c *chart.Chart, t time.Time) {
+	c.ModTime = t
+	c.SchemaModTime = t
+	for _, f := range c.Raw {
+		f.ModTime = t
+	}
+	for _, f := range c.Templates {
+		f.ModTime = t
+	}
+	for _, f := range c.Files {
+		f.ModTime = t
+	}
+	for _, dep := range c.Dependencies() {
+		stampModTimes(dep, t)
+	}
+}

pkg/action/package_test.go

@@ -17,10 +17,14 @@ limitations under the License.
 package action
 
 import (
+	"archive/tar"
+	"compress/gzip"
 	"errors"
+	"io"
 	"os"
 	"path"
 	"testing"
+	"time"
 
 	"github.com/Masterminds/semver/v3"
 	"github.com/stretchr/testify/require"
@@ -170,3 +174,56 @@ func TestRun(t *testing.T) {
 	require.Equal(t, "empty-0.1.0.tgz", filename)
 	require.NoError(t, os.Remove(filename))
 }
+
+func TestRunWithSourceDateEpoch(t *testing.T) {
+	chartPath := "testdata/charts/chart-with-schema"
+	epoch := time.Unix(1700000000, 0)
+
+	client := NewPackage()
+	client.SourceDateEpoch = &epoch
+
+	filename, err := client.Run(chartPath, nil)
+	require.NoError(t, err)
+	t.Cleanup(func() { os.Remove(filename) })
+
+	// All tar entry ModTimes must equal the epoch.
+	f, err := os.Open(filename)
+	require.NoError(t, err)
+	defer f.Close()
+
+	gr, err := gzip.NewReader(f)
+	require.NoError(t, err)
+	defer gr.Close()
+
+	tr := tar.NewReader(gr)
+	for {
+		hdr, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		require.NoError(t, err)
+		require.Equal(t, epoch, hdr.ModTime, "expected epoch ModTime for entry %s", hdr.Name)
+	}
+}
+
+func TestRunWithSourceDateEpochReproducible(t *testing.T) {
+	chartPath := "testdata/charts/chart-with-schema"
+	epoch := time.Unix(1700000000, 0)
+
+	build := func() []byte {
+		t.Helper()
+		dir := t.TempDir()
+		client := NewPackage()
+		client.SourceDateEpoch = &epoch
+		client.Destination = dir
+		filename, err := client.Run(chartPath, nil)
+		require.NoError(t, err)
+		data, err := os.ReadFile(filename)
+		require.NoError(t, err)
+		return data
+	}
+
+	first := build()
+	second := build()
+	require.Equal(t, first, second, "two builds with the same SOURCE_DATE_EPOCH must be byte-identical")
+}

pkg/cmd/package.go

@@ -22,6 +22,8 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strconv"
+	"time"
 
 	"github.com/spf13/cobra"
 
@@ -59,6 +61,14 @@ func newPackageCmd(out io.Writer) *cobra.Command {
 			if len(args) == 0 {
 				return errors.New("need at least one argument, the path to the chart")
 			}
+			if epochStr, ok := os.LookupEnv("SOURCE_DATE_EPOCH"); ok {
+				sec, err := strconv.ParseInt(epochStr, 10, 64)
+				if err != nil || sec < 0 {
+					return fmt.Errorf("invalid SOURCE_DATE_EPOCH %q: must be a non-negative Unix timestamp", epochStr)
+				}
+				t := time.Unix(sec, 0)
+				client.SourceDateEpoch = &t
+			}
 			if client.Sign {
 				if client.Key == "" {
 					return errors.New("--key is required for signing a package")