archive/tar: add support for atime and ctime to Writer

Both the GNU and PAX formats support atime and ctime fields.
The implementation is trivial now that we have:
* support for formatting PAX records for timestamps
* dedicated methods that only handle one format (e.g., GNU)

Fixes #17876

Change-Id: I0c604fce14a47d722098afc966399cca2037395d
Reviewed-on: https://go-review.googlesource.com/55570
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/archive/tar/common.go

@@ -133,8 +133,9 @@ func (h *Header) allowedFormats() (format int, paxHdrs map[string]string) {
 	const supportGNULong = false
 
 	var blk block
-	var v7 = blk.V7()
-	var ustar = blk.USTAR()
+	v7 := blk.V7()
+	ustar := blk.USTAR()
+	gnu := blk.GNU()
 	verifyString(h.Name, len(v7.Name()), supportGNULong, paxPath)
 	verifyString(h.Linkname, len(v7.LinkName()), supportGNULong, paxLinkpath)
 	verifyString(h.Uname, len(ustar.UserName()), false, paxUname)
@@ -146,9 +147,8 @@ func (h *Header) allowedFormats() (format int, paxHdrs map[string]string) {
 	verifyNumeric(h.Devmajor, len(ustar.DevMajor()), paxNone)
 	verifyNumeric(h.Devminor, len(ustar.DevMinor()), paxNone)
 	verifyTime(h.ModTime, len(v7.ModTime()), true, paxMtime)
-	// TODO(dsnet): Support atime and ctime fields.
-	// verifyTime(h.AccessTime, len(gnu.AccessTime()), false, paxAtime)
-	// verifyTime(h.ChangeTime, len(gnu.ChangeTime()), false, paxCtime)
+	verifyTime(h.AccessTime, len(gnu.AccessTime()), false, paxAtime)
+	verifyTime(h.ChangeTime, len(gnu.ChangeTime()), false, paxCtime)
 
 	if !isHeaderOnlyType(h.Typeflag) && h.Size < 0 {
 		return formatUnknown, nil

src/archive/tar/tar_test.go

@@ -450,6 +450,34 @@ func TestHeaderAllowedFormats(t *testing.T) {
 	}, {
 		header:  &Header{ModTime: time.Unix(077777777777, 0)},
 		formats: formatUSTAR | formatPAX | formatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(077777777777+1, 0)},
+		paxHdrs: map[string]string{paxMtime: "8589934592"},
+		formats: formatPAX | formatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(math.MaxInt64, 0)},
+		paxHdrs: map[string]string{paxMtime: "9223372036854775807"},
+		formats: formatPAX | formatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(-1, 0)},
+		paxHdrs: map[string]string{paxMtime: "-1"},
+		formats: formatPAX | formatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(-1, 500)},
+		paxHdrs: map[string]string{paxMtime: "-0.9999995"},
+		formats: formatPAX,
+	}, {
+		header:  &Header{AccessTime: time.Unix(0, 0)},
+		paxHdrs: map[string]string{paxAtime: "0"},
+		formats: formatPAX | formatGNU,
+	}, {
+		header:  &Header{AccessTime: time.Unix(-123, 0)},
+		paxHdrs: map[string]string{paxAtime: "-123"},
+		formats: formatPAX | formatGNU,
+	}, {
+		header:  &Header{ChangeTime: time.Unix(123, 456)},
+		paxHdrs: map[string]string{paxCtime: "123.000000456"},
+		formats: formatPAX,
 	}}
 
 	for i, v := range vectors {

src/archive/tar/writer.go

@@ -141,8 +141,12 @@ func (tw *Writer) writeGNUHeader(hdr *Header) error {
 	// Pack the main header.
 	var f formatter
 	blk := tw.templateV7Plus(hdr, f.formatString, f.formatNumeric)
-	// TODO(dsnet): Support atime and ctime fields.
-	// See https://golang.org/issue/17876
+	if !hdr.AccessTime.IsZero() {
+		f.formatNumeric(blk.GNU().AccessTime(), hdr.AccessTime.Unix())
+	}
+	if !hdr.ChangeTime.IsZero() {
+		f.formatNumeric(blk.GNU().ChangeTime(), hdr.ChangeTime.Unix())
+	}
 	blk.SetFormat(formatGNU)
 	if f.err != nil {
 		return f.err // Should never happen since header is validated