archive/tar: doesn't generate atime entries

[cyphar]

% go version
go version go1.7 linux/amd64
% go env
GOARCH="amd64"
GOBIN="/home/cyphar/go/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/cyphar/.local"
GORACE=""
GOROOT="/usr/lib64/go"
GOTOOLDIR="/usr/lib64/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build013320516=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

This also happens on https://play.golang.org/, which you can see in this snippet: https://play.golang.org/p/QURChe1JI_.

package main

import (
        "archive/tar"
        "fmt"
        "io"
        "time"
)

func main() {
        reader, writer := io.Pipe()

        atime := time.Unix(123, 0)
        mtime := time.Unix(777, 0)
        ctime := time.Unix(1337, 0)

        tw := tar.NewWriter(writer)
        tr := tar.NewReader(reader)

        go func() {
                tw.WriteHeader(&tar.Header{
                        Name:       "somefile",
                        Mode:       0777,
                        ModTime:    mtime,
                        AccessTime: atime,
                        ChangeTime: ctime,
                })
                tw.Close()
        }()

        hdr, err := tr.Next()
        if err != nil {
                panic(err)
        }

        if !hdr.ModTime.Equal(mtime) {
                fmt.Printf("mtime not equal! got %s, expected %s\n", hdr.ModTime, mtime)
        }
        if !hdr.AccessTime.Equal(atime) {
                fmt.Printf("atime not equal! got %s, expected %s\n", hdr.AccessTime, atime)
        }
        if !hdr.ChangeTime.Equal(ctime) {
                fmt.Printf("ctime not equal! got %s, expected %s\n", hdr.ChangeTime, ctime)
        }
}

What did you expect to see?

No errors in the above program.

What did you see instead?

% go run broken.go
atime not equal! got 0001-01-01 00:00:00 +0000 UTC, expected 1970-01-01 10:02:03 +1000 AEST
ctime not equal! got 0001-01-01 00:00:00 +0000 UTC, expected 1970-01-01 10:22:17 +1000 AEST

From what I can tell, this is caused by the fact that WriteHeader doesn't in any way touch the AccessTime of the structure. The same goes for the ChangeTime.

[cyphar]

I played with this a bit, and it looks like the issue is that currently we're generating USTAR headers, but only GNU and STAR have support for those headers. This was my attempt, but it obviously doesn't work:

diff --git a/src/archive/tar/writer.go b/src/archive/tar/writer.go
index 596fb8b9e171..0d866d3fbf4a 100644
--- a/src/archive/tar/writer.go
+++ b/src/archive/tar/writer.go
@@ -145,11 +145,17 @@ func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
                f.formatNumeric(b, x)
        }

-       // Handle out of range ModTime carefully.
-       var modTime int64
+       // Handle out of range ModTime, AccessTime, and ChangeTime carefully.
+       var modTime, accessTime, changeTime int64
        if !hdr.ModTime.Before(minTime) && !hdr.ModTime.After(maxTime) {
                modTime = hdr.ModTime.Unix()
        }
+       if !hdr.AccessTime.Before(minTime) && !hdr.AccessTime.After(maxTime) {
+               accessTime = hdr.AccessTime.Unix()
+       }
+       if !hdr.ChangeTime.Before(minTime) && !hdr.ChangeTime.After(maxTime) {
+               changeTime = hdr.ChangeTime.Unix()
+       }

        v7 := header.V7()
        formatString(v7.Name(), hdr.Name, paxPath)
@@ -169,6 +175,9 @@ func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
        formatString(ustar.GroupName(), hdr.Gname, paxGname)
        formatNumeric(ustar.DevMajor(), hdr.Devmajor, paxNone)
        formatNumeric(ustar.DevMinor(), hdr.Devminor, paxNone)
+       // TODO: This doesn't work because we output USTAR rather than STAR or GNU.
+       //formatNumeric(ustar.AccessTime(), accessTime, paxAtime)
+       //formatNumeric(utar.ChangeTime(), changeTime, paxCtime)

        // TODO(dsnet): The logic surrounding the prefix field is broken when trying
        // to encode the header as GNU format. The challenge with the current logic

[dsnet]

I plan on overhauling the Writer logic in Go 1.9 and I have a fix for this. Thanks for the report.

[gopherbot]

Change https://golang.org/cl/55570 mentions this issue: archive/tar: add support for atime and ctime to Writer

[gopherbot]

Change https://golang.org/cl/59230 mentions this issue: archive/tar: require opt-in to PAX or GNU format for time features