fix(chart): format the v3 gzip extra field as an RFC 1952 subfield

internal/chart/v3/util/save.go assigned gzip.Writer.Extra the 41-byte
string "+aHR0cHM6Ly95b3V0dS5iZS96OVV6MWljandyTQo=", which is not a valid
FEXTRA payload. RFC 1952 section 2.3.1.1 requires every subfield to be
SI1, SI2, a 2-byte little-endian LEN, and then LEN bytes of data. Parsed
that way those bytes give SI1='+', SI2='a' and LEN=21064 while only 37
bytes remain, so strict readers reject every archive the v3 writer
produces. Go's compress/gzip does not parse subfields, so nothing in the
tree noticed.

pkg/chart/v2/util/save.go had the identical defect. It was fixed in
#31884 for issue #31844, "helm package produces malformed .tgz", where
the Bazel downloader failed with "Extra subfield lenght exceeds
remaining bytes in extra: 21064 > 37". The v3 writer was never updated.

Use the same header bytes the v2 writer now uses: SI1 and SI2 of 'r',
LEN 0x0028, and the unchanged 40-byte payload. Both writers now emit
byte-identical, well-formed extra fields. Adds the v2 regression test to
the v3 package.

Signed-off-by: Max Freedom Pollard <272618364+MaxFreedomPollard@users.noreply.github.com>

internal/chart/v3/util/save.go

@@ -33,7 +33,11 @@ import (
 	"helm.sh/helm/v4/pkg/chart/common"
 )
 
-var headerBytes = []byte("+aHR0cHM6Ly95b3V0dS5iZS96OVV6MWljandyTQo=")
+// RFC 1952 subfield header:
+// +---+---+---+---+==================================+
+// |SI1|SI2|  LEN  |... LEN bytes of subfield data ...|
+// +---+---+---+---+==================================+
+var headerBytes = []byte("rr\x28\x00aHR0cHM6Ly95b3V0dS5iZS96OVV6MWljandyTQo=")
 
 // SaveDir saves a chart as files in a directory.
 //

internal/chart/v3/util/save_test.go

@@ -21,6 +21,7 @@ import (
 	"bytes"
 	"compress/gzip"
 	"crypto/sha256"
+	"encoding/binary"
 	"encoding/hex"
 	"errors"
 	"io"
@@ -108,6 +109,41 @@ func TestSave(t *testing.T) {
 	require.Error(t, err, "Expected error saving chart with invalid name")
 }
 
+func TestSavedGzipExtraFieldIsValid(t *testing.T) {
+	tmp := t.TempDir()
+	c := &chart.Chart{
+		Metadata: &chart.Metadata{
+			APIVersion: chart.APIVersionV3,
+			Name:       "ahab",
+			Version:    "1.2.3",
+		},
+	}
+
+	where, err := Save(c, tmp)
+	require.NoError(t, err, "Failed to save")
+
+	f, err := os.Open(where)
+	require.NoError(t, err, "Failed to open saved file")
+	defer f.Close()
+
+	r, err := gzip.NewReader(f)
+	require.NoError(t, err, "Failed to create gzip reader")
+	defer r.Close()
+
+	// RFC 1952 §2.3.1.1:
+	// Each subfield consists of SI1, SI2 (1 byte each),
+	// a 2-byte little-endian LEN, and LEN bytes of data.
+	// https://www.rfc-editor.org/rfc/rfc1952.html#page-8
+	extra := r.Extra
+
+	require.NotEmpty(t, extra)
+	require.GreaterOrEqual(t, len(extra), 4)
+
+	dataLen := int(binary.LittleEndian.Uint16(extra[2:4]))
+	// Assume a single subfield.
+	require.Lenf(t, extra, 4+dataLen, "gzip extra field has malformed subfield: LEN=%d but %d data byte(s) follow the subfield header", dataLen, len(extra)-4)
+}
+
 // Creates a copy with a different schema; does not modify anything.
 func withSchema(chart chart.Chart, schema []byte) chart.Chart {
 	chart.Schema = schema