archive/zip: handle extra data headers with no body

Fixes #4393.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6854058

src/pkg/archive/zip/reader.go

@@ -238,7 +238,7 @@ func readDirectoryHeader(f *File, r io.Reader) error {
 
 	if len(f.Extra) > 0 {
 		b := readBuf(f.Extra)
-		for len(b) > 4 { // need at least tag and size
+		for len(b) >= 4 { // need at least tag and size
 			tag := b.uint16()
 			size := b.uint16()
 			if int(size) > len(b) {

src/pkg/archive/zip/zip_test.go

@@ -195,6 +195,27 @@ func testInvalidHeader(h *FileHeader, t *testing.T) {
 	}
 }
 
+func testValidHeader(h *FileHeader, t *testing.T) {
+	var buf bytes.Buffer
+	z := NewWriter(&buf)
+
+	f, err := z.CreateHeader(h)
+	if err != nil {
+		t.Fatalf("error creating header: %v", err)
+	}
+	if _, err := f.Write([]byte("hi")); err != nil {
+		t.Fatalf("error writing content: %v", err)
+	}
+	if err := z.Close(); err != nil {
+		t.Fatalf("error closing zip writer: %v", err)
+	}
+
+	b := buf.Bytes()
+	if _, err = NewReader(bytes.NewReader(b), int64(len(b))); err != nil {
+		t.Fatalf("got %v, expected nil", err)
+	}
+}
+
 // Issue 4302.
 func TestHeaderInvalidTagAndSize(t *testing.T) {
 	const timeFormat = "20060102T150405.000.txt"
@@ -220,3 +241,17 @@ func TestHeaderTooShort(t *testing.T) {
 	}
 	testInvalidHeader(&h, t)
 }
+
+// Issue 4393. It is valid to have an extra data header
+// which contains no body.
+func TestZeroLengthHeader(t *testing.T) {
+	h := FileHeader{
+		Name:   "extadata.txt",
+		Method: Deflate,
+		Extra: []byte{
+			85, 84, 5, 0, 3, 154, 144, 195, 77, // tag 21589 size 5
+			85, 120, 0, 0, // tag 30805 size 0
+		},
+	}
+	testValidHeader(&h, t)
+}