archive/zip: only use Extended Timestamp on non-zero MS-DOS timestamps

We should preserve the fact that a roundtrip read on fields with the zero
value should remain the zero for those that are reasonable to stay that way.
If the zero value for a MS-DOS timestamp was used, then it is sensible for
that zero value to also be read back later.

Fixes #17403

Change-Id: I32c3915eab180e91ddd2499007374f7b85f0bd76
Reviewed-on: https://go-review.googlesource.com/30811
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/zip/struct.go

@@ -65,7 +65,7 @@ const (
 	zip64ExtraId = 0x0001 // zip64 Extended Information Extra Field
 	ntfsExtraId  = 0x000a // NTFS Extra Field
 	unixExtraId  = 0x000d // UNIX Extra Field
-	exttsExtraId = 0x5455 // Extra Timestamp Extra Field
+	exttsExtraId = 0x5455 // Extended Timestamp Extra Field
 )
 
 // FileHeader describes a file within a zip file.

src/archive/zip/writer.go

@@ -99,14 +99,17 @@ func (w *Writer) Close() error {
 			b.uint32(h.UncompressedSize)
 		}
 
-		mt := uint32(h.FileHeader.ModTime().Unix())
-		var mbuf [9]byte // 2x uint16 + uint8 + uint32
-		eb := writeBuf(mbuf[:])
-		eb.uint16(exttsExtraId)
-		eb.uint16(5)  // size = uint8 + uint32
-		eb.uint8(1)   // flags = modtime
-		eb.uint32(mt) // ModTime
-		h.Extra = append(h.Extra, mbuf[:]...)
+		// use Extended Timestamp Extra Field.
+		if h.ModifiedTime != 0 || h.ModifiedDate != 0 {
+			mt := uint32(h.ModTime().Unix())
+			var mbuf [9]byte // 2x uint16 + uint8 + uint32
+			eb := writeBuf(mbuf[:])
+			eb.uint16(exttsExtraId)
+			eb.uint16(5)  // size = uint8 + uint32
+			eb.uint8(1)   // flags = modtime
+			eb.uint32(mt) // ModTime
+			h.Extra = append(h.Extra, mbuf[:]...)
+		}
 
 		b.uint16(uint16(len(h.Name)))
 		b.uint16(uint16(len(h.Extra)))

src/archive/zip/zip_test.go

@@ -13,6 +13,7 @@ import (
 	"internal/testenv"
 	"io"
 	"io/ioutil"
+	"reflect"
 	"sort"
 	"strings"
 	"testing"
@@ -111,6 +112,44 @@ func TestFileHeaderRoundTrip64(t *testing.T) {
 	testHeaderRoundTrip(fh, uint32max, fh.UncompressedSize64, t)
 }
 
+func TestZeroFileRoundTrip(t *testing.T) {
+	var b bytes.Buffer
+	w := NewWriter(&b)
+	if _, err := w.Create(""); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+	r, err := NewReader(bytes.NewReader(b.Bytes()), int64(b.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify that fields that should reasonably be the zero value stays
+	// as the zero value.
+	var want FileHeader
+	if len(r.File) != 1 {
+		t.Fatalf("len(r.File) = %d, want 1", len(r.File))
+	}
+	fh := r.File[0].FileHeader
+	got := FileHeader{
+		Name:               fh.Name,
+		ModifiedTime:       fh.ModifiedTime,
+		ModifiedDate:       fh.ModifiedDate,
+		UncompressedSize:   fh.UncompressedSize,
+		UncompressedSize64: fh.UncompressedSize64,
+		ExternalAttrs:      fh.ExternalAttrs,
+		Comment:            fh.Comment,
+	}
+	if len(fh.Extra) > 0 {
+		got.Extra = fh.Extra
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("FileHeader mismatch:\ngot  %#v\nwant %#v", got, want)
+	}
+}
+
 type repeatedByte struct {
 	off int64
 	b   byte