archive/tar: support PAX subsecond resolution times

Add support for PAX subsecond resolution times. Since the parser
supports negative timestamps, the formatter also handles negative
timestamps.

The relevant PAX specification is:
<<<
Portable file timestamps cannot be negative. If pax encounters a
file with a negative timestamp in copy or write mode, it can reject
the file, substitute a non-negative timestamp, or generate a
non-portable timestamp with a leading '-'.
>>>

<<<
All of these time records shall be formatted as a decimal
representation of the time in seconds since the Epoch.
If a <period> ( '.' ) decimal point character is present,
the digits to the right of the point shall represent the units of
a subsecond timing granularity, where the first digit is tenths of
a second and each subsequent digit is a tenth of the previous digit.
>>>

Fixes #11171

Change-Id: Ied108f3d2654390bc1b0ddd66a4081c2b83e490b
Reviewed-on: https://go-review.googlesource.com/55552
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

4 files changed, 68 insertions, 11 deletions

diff --git a/src/archive/tar/common.go b/src/archive/tar/common.go
index 5b7bbe5a1e..5b921486f1 100644
--- a/src/archive/tar/common.go
+++ b/src/archive/tar/common.go
@@ -124,8 +124,7 @@ func (h *Header) allowedFormats() (format int, paxHdrs map[string]string) {
 			if paxKey == paxNone {
 				format &^= formatPAX // No PAX
 			} else {
-				// TODO(dsnet): Support PAX time here.
-				// paxHdrs[paxKey] = formatPAXTime(ts)
+				paxHdrs[paxKey] = formatPAXTime(ts)
 			}
 		}
 	}
diff --git a/src/archive/tar/strconv.go b/src/archive/tar/strconv.go
index 6aad6805aa..a93fc4ac7a 100644
--- a/src/archive/tar/strconv.go
+++ b/src/archive/tar/strconv.go
@@ -6,6 +6,7 @@ package tar
 
 import (
 	"bytes"
+	"fmt"
 	"strconv"
 	"strings"
 	"time"
@@ -218,7 +219,23 @@ func parsePAXTime(s string) (time.Time, error) {
 	return time.Unix(secs, int64(nsecs)), nil
 }
 
-// TODO(dsnet): Implement formatPAXTime.
+// formatPAXTime converts ts into a time of the form %d.%d as described in the
+// PAX specification. This function is capable of negative timestamps.
+func formatPAXTime(ts time.Time) (s string) {
+	secs, nsecs := ts.Unix(), ts.Nanosecond()
+	if nsecs == 0 {
+		return strconv.FormatInt(secs, 10)
+	}
+
+	// If seconds is negative, then perform correction.
+	sign := ""
+	if secs < 0 {
+		sign = "-"             // Remember sign
+		secs = -(secs + 1)     // Add a second to secs
+		nsecs = -(nsecs - 1E9) // Take that second away from nsecs
+	}
+	return strings.TrimRight(fmt.Sprintf("%s%d.%09d", sign, secs, nsecs), "0")
+}
 
 // parsePAXRecord parses the input PAX record string into a key-value pair.
 // If parsing is successful, it will slice off the currently read record and
diff --git a/src/archive/tar/strconv_test.go b/src/archive/tar/strconv_test.go
index e2527dc61f..7156368ede 100644
--- a/src/archive/tar/strconv_test.go
+++ b/src/archive/tar/strconv_test.go
@@ -294,6 +294,51 @@ func TestParsePAXTime(t *testing.T) {
 	}
 }
 
+func TestFormatPAXTime(t *testing.T) {
+	vectors := []struct {
+		sec, nsec int64
+		want      string
+	}{
+		{1350244992, 0, "1350244992"},
+		{1350244992, 300000000, "1350244992.3"},
+		{1350244992, 23960100, "1350244992.0239601"},
+		{1350244992, 23960108, "1350244992.023960108"},
+		{+1, +1E9 - 1E0, "1.999999999"},
+		{+1, +1E9 - 1E3, "1.999999"},
+		{+1, +1E9 - 1E6, "1.999"},
+		{+1, +0E0 - 0E0, "1"},
+		{+1, +1E6 - 0E0, "1.001"},
+		{+1, +1E3 - 0E0, "1.000001"},
+		{+1, +1E0 - 0E0, "1.000000001"},
+		{0, 1E9 - 1E0, "0.999999999"},
+		{0, 1E9 - 1E3, "0.999999"},
+		{0, 1E9 - 1E6, "0.999"},
+		{0, 0E0, "0"},
+		{0, 1E6 + 0E0, "0.001"},
+		{0, 1E3 + 0E0, "0.000001"},
+		{0, 1E0 + 0E0, "0.000000001"},
+		{-1, -1E9 + 1E0, "-1.999999999"},
+		{-1, -1E9 + 1E3, "-1.999999"},
+		{-1, -1E9 + 1E6, "-1.999"},
+		{-1, -0E0 + 0E0, "-1"},
+		{-1, -1E6 + 0E0, "-1.001"},
+		{-1, -1E3 + 0E0, "-1.000001"},
+		{-1, -1E0 + 0E0, "-1.000000001"},
+		{-1350244992, 0, "-1350244992"},
+		{-1350244992, -300000000, "-1350244992.3"},
+		{-1350244992, -23960100, "-1350244992.0239601"},
+		{-1350244992, -23960108, "-1350244992.023960108"},
+	}
+
+	for _, v := range vectors {
+		got := formatPAXTime(time.Unix(v.sec, v.nsec))
+		if got != v.want {
+			t.Errorf("formatPAXTime(%ds, %dns): got %q, want %q",
+				v.sec, v.nsec, got, v.want)
+		}
+	}
+}
+
 func TestParsePAXRecord(t *testing.T) {
 	medName := strings.Repeat("CD", 50)
 	longName := strings.Repeat("AB", 100)
diff --git a/src/archive/tar/writer.go b/src/archive/tar/writer.go
index d88e5ef340..7d393b46df 100644
--- a/src/archive/tar/writer.go
+++ b/src/archive/tar/writer.go
@@ -67,17 +67,13 @@ func (tw *Writer) WriteHeader(hdr *Header) error {
 		return err
 	}
 
-	// TODO(dsnet): Add PAX timestamps with nanosecond support.
-	hdrCpy := *hdr
-	hdrCpy.ModTime = hdrCpy.ModTime.Truncate(time.Second)
-
-	switch allowedFormats, paxHdrs := hdrCpy.allowedFormats(); {
+	switch allowedFormats, paxHdrs := hdr.allowedFormats(); {
 	case allowedFormats&formatUSTAR != 0:
-		return tw.writeUSTARHeader(&hdrCpy)
+		return tw.writeUSTARHeader(hdr)
 	case allowedFormats&formatPAX != 0:
-		return tw.writePAXHeader(&hdrCpy, paxHdrs)
+		return tw.writePAXHeader(hdr, paxHdrs)
 	case allowedFormats&formatGNU != 0:
-		return tw.writeGNUHeader(&hdrCpy)
+		return tw.writeGNUHeader(hdr)
 	default:
 		return ErrHeader
 	}