Fix v3 gzip extra field to use a valid RFC 1952 subfield

The chart v3 save path was writing raw marker bytes into the gzip `Extra`
field, which produced malformed subfield metadata in readers that validate
RFC 1952 framing.

Wrap Helm's existing marker payload in a proper gzip subfield header
(`SI1 SI2 LEN DATA`) and keep the payload unchanged. Reject oversized
subfield payloads before encoding the two-byte gzip length.

Add regression coverage for the extra field framing, oversized payload guard,
and repeatable save hashes.

Tested: go test ./internal/chart/v3/util -run TestSavedGzipExtraFieldIsValid -count=1 (Go 1.26 container)
Tested: go test ./internal/chart/v3/util -run TestGzipExtraFieldWithSubfieldRejectsOversizedPayload -count=1 (Go 1.26 container)
Tested: go test ./internal/chart/v3/util -run TestRepeatableSave -count=1 (Go 1.26 container)
Tested: go test ./internal/chart/v3/util -run 'TestSavedGzipExtraFieldIsValid|TestGzipExtraFieldWithSubfieldRejectsOversizedPayload|TestSave|TestRepeatableSave' -count=1 (Go 1.26 container)
Signed-off-by: Mike Ma <mike.ma@lmctl.com>

internal/chart/v3/util/save.go

@@ -19,6 +19,7 @@ package util
 import (
 	"archive/tar"
 	"compress/gzip"
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -34,6 +35,22 @@ import (
 )
 
 var headerBytes = []byte("+aHR0cHM6Ly95b3V0dS5iZS96OVV6MWljandyTQo=")
+var gzipExtraSubfieldID = [2]byte{'r', 'r'}
+
+const maxGzipExtraSubfieldPayloadLen = 1<<16 - 1
+
+func gzipExtraFieldWithSubfield(payload []byte) ([]byte, error) {
+	if len(payload) > maxGzipExtraSubfieldPayloadLen {
+		return nil, fmt.Errorf("gzip extra subfield payload too large: %d bytes", len(payload))
+	}
+
+	extra := make([]byte, 4+len(payload))
+	extra[0] = gzipExtraSubfieldID[0]
+	extra[1] = gzipExtraSubfieldID[1]
+	binary.LittleEndian.PutUint16(extra[2:4], uint16(len(payload)))
+	copy(extra[4:], payload)
+	return extra, nil
+}
 
 // SaveDir saves a chart as files in a directory.
 //
@@ -125,6 +142,11 @@ func Save(c *chart.Chart, outDir string) (string, error) {
 		return "", fmt.Errorf("is not a directory: %s", dir)
 	}
 
+	gzipExtra, err := gzipExtraFieldWithSubfield(headerBytes)
+	if err != nil {
+		return "", err
+	}
+
 	f, err := os.Create(filename)
 	if err != nil {
 		return "", err
@@ -132,7 +154,7 @@ func Save(c *chart.Chart, outDir string) (string, error) {
 
 	// Wrap in gzip writer
 	zipper := gzip.NewWriter(f)
-	zipper.Extra = headerBytes
+	zipper.Extra = gzipExtra
 	zipper.Comment = "Helm"
 
 	// Wrap in tar writer

internal/chart/v3/util/save_test.go

@@ -127,6 +127,58 @@ func TestSave(t *testing.T) {
 	}
 }
 
+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)
+	if err != nil {
+		t.Fatalf("Failed to save: %s", err)
+	}
+
+	f, err := os.Open(where)
+	if err != nil {
+		t.Fatalf("Failed to open saved file: %s", err)
+	}
+	defer f.Close()
+
+	r, err := gzip.NewReader(f)
+	if err != nil {
+		t.Fatalf("Failed to create gzip reader: %s", err)
+	}
+	defer r.Close()
+
+	if len(r.Extra) < 4 {
+		t.Fatalf("gzip extra field too short to contain a valid subfield: %d byte(s)", len(r.Extra))
+	}
+
+	dataLen := int(r.Extra[2]) | int(r.Extra[3])<<8
+	if len(r.Extra) != 4+dataLen {
+		t.Fatalf("gzip extra field has malformed subfield: LEN=%d but %d data byte(s) follow the subfield header", dataLen, len(r.Extra)-4)
+	}
+
+	if r.Extra[0] != gzipExtraSubfieldID[0] || r.Extra[1] != gzipExtraSubfieldID[1] {
+		t.Fatalf("gzip extra field has unexpected subfield ID: %q%q", r.Extra[0], r.Extra[1])
+	}
+
+	if !bytes.Equal(r.Extra[4:], headerBytes) {
+		t.Fatalf("gzip extra field payload mismatch")
+	}
+}
+
+func TestGzipExtraFieldWithSubfieldRejectsOversizedPayload(t *testing.T) {
+	_, err := gzipExtraFieldWithSubfield(make([]byte, maxGzipExtraSubfieldPayloadLen+1))
+	if err == nil {
+		t.Fatal("expected oversized gzip extra subfield payload to fail")
+	}
+}
+
 // Creates a copy with a different schema; does not modify anything.
 func withSchema(chart chart.Chart, schema []byte) chart.Chart {
 	chart.Schema = schema
@@ -295,7 +347,7 @@ func TestRepeatableSave(t *testing.T) {
 				Schema:        []byte("{\n  \"title\": \"Values\"\n}"),
 				SchemaModTime: modTime,
 			},
-			want: "5bfea18cc3c8cbc265744bc32bffa9489a4dbe87d6b51b90f4255e4839d35e03",
+			want: "47f62cfbd69445112e92170ce8fc607c868e221045a1ce0696ac0df68841615f",
 		},
 		{
 			name: "Package 2 files",
@@ -317,7 +369,7 @@ func TestRepeatableSave(t *testing.T) {
 				Schema:        []byte("{\n  \"title\": \"Values\"\n}"),
 				SchemaModTime: modTime,
 			},
-			want: "a240365c21e0a2f4a57873132a9b686566a612d08bcb3f20c9446bfff005ccce",
+			want: "58692b29601b75e1a0cc16d39d48ac2b71f45e05812f8d421a2cede39291fe93",
 		},
 	}
 	for _, test := range tests {