50% chance Mtim field doesn't get updated

Mtim is updated

root@e25213faeb41:/go/src/github.com/docker/docker# go test -v github.com/docker/docker/pkg/archive -run TestChangesDirsMutated
=== RUN TestChangesDirsMutated
--- PASS: TestChangesDirsMutated (0.11 seconds)
	changes_test.go:208: Lstat(old): &{name:symlink2 size:7 mode:134218239 modTime:{sec:63551388019 nsec:33650046 loc:0x6f8f40} sys:0xc208048510}
	changes_test.go:209: Lstat(new): &{name:symlink2 size:13 mode:134218239 modTime:{sec:63551388019 nsec:43652546 loc:0x6f8f40} sys:0xc208048630}
	changes_test.go:210: sameFsTime=false, expected=false
	changes_test.go:212: stat(old): &{Dev:34 Ino:171198 Nlink:1 Mode:41471 Uid:0 Gid:0 X__pad0:0 Rdev:0 Size:7 Blksize:4096 Blocks:0 Atim:{Sec:1415791219 Nsec:33650046} Mtim:{Sec:1415791219 Nsec:33650046} Ctim:{Sec:1415791219 Nsec:43652546} X__unused:[0 0 0]}
	changes_test.go:213: stat(new): &{Dev:34 Ino:172134 Nlink:1 Mode:41471 Uid:0 Gid:0 X__pad0:0 Rdev:0 Size:13 Blksize:4096 Blocks:0 Atim:{Sec:1415791219 Nsec:43652546} Mtim:{Sec:1415791219 Nsec:43652546} Ctim:{Sec:1415791219 Nsec:43652546} X__unused:[0 0 0]}
	changes_test.go:214: sameFsTimespec=false, expected=false
PASS

symlink updated but Mtim is same?!

=== RUN TestChangesDirsMutated
--- FAIL: TestChangesDirsMutated (0.11 seconds)
	changes_test.go:208: Lstat(old): &{name:symlink2 size:7 mode:134218239 modTime:{sec:63551388097 nsec:493260046 loc:0x6f8f40} sys:0xc208048510}
	changes_test.go:209: Lstat(new): &{name:symlink2 size:13 mode:134218239 modTime:{sec:63551388097 nsec:493260046 loc:0x6f8f40} sys:0xc208048630}
	changes_test.go:210: sameFsTime=true, expected=false
	changes_test.go:212: stat(old): &{Dev:34 Ino:172162 Nlink:1 Mode:41471 Uid:0 Gid:0 X__pad0:0 Rdev:0 Size:7 Blksize:4096 Blocks:0 Atim:{Sec:1415791297 Nsec:493260046} Mtim:{Sec:1415791297 Nsec:493260046} Ctim:{Sec:1415791297 Nsec:493260046} X__unused:[0 0 0]}
	changes_test.go:213: stat(new): &{Dev:34 Ino:170571 Nlink:1 Mode:41471 Uid:0 Gid:0 X__pad0:0 Rdev:0 Size:13 Blksize:4096 Blocks:0 Atim:{Sec:1415791297 Nsec:493260046} Mtim:{Sec:1415791297 Nsec:493260046} Ctim:{Sec:1415791297 Nsec:493260046} X__unused:[0 0 0]}
	changes_test.go:214: sameFsTimespec=true, expected=false
	changes_test.go:286: no change for expected change C /symlink2 != A /symlinknew
FAIL
exit status 1
FAIL	github.com/docker/docker/pkg/archive	0.119s

test.diff

diff --git a/pkg/archive/changes_test.go b/pkg/archive/changes_test.go
index 34c0f0d..3b10507 100644
--- a/pkg/archive/changes_test.go
+++ b/pkg/archive/changes_test.go
@@ -1,6 +1,7 @@
 package archive

 import (
+       "syscall"
        "io/ioutil"
        "os"
        "os/exec"
@@ -179,6 +180,14 @@ func mutateSampleDir(t *testing.T, root string) {
        }

        // Change a symlink
+       stat1,err := os.Lstat(path.Join(root, "symlink2"))
+       if err != nil { t.Fatal(err) }
+
+       s1 := &syscall.Stat_t{}
+       err = syscall.Lstat(path.Join(root, "symlink2"), s1)
+       if err != nil { t.Fatal(err) }
+       ss1 := FromStat_t(s1)
+
        if err := os.RemoveAll(path.Join(root, "symlink2")); err != nil {
                t.Fatal(err)
        }
@@ -186,6 +195,23 @@ func mutateSampleDir(t *testing.T, root string) {
                t.Fatal(err)
        }

+       stat2, err := os.Lstat(path.Join(root, "symlink2"))
+       if err != nil { t.Fatal(err) }
+
+       s2 := &syscall.Stat_t{}
+       err = syscall.Lstat(path.Join(root, "symlink2"), s2)
+       if err != nil { t.Fatal(err) }
+       ss2 := FromStat_t(s2)
+
+       t.Logf("Lstat(old): %+v", stat1)
+       t.Logf("Lstat(new): %+v", stat2)
+       t.Logf("sameFsTime=%v, expected=false\n", sameFsTime(stat1.ModTime(), stat2.ModTime()))
+
+       t.Logf("stat(old): %+v", s1)
+       t.Logf("stat(new): %+v", s2)
+       t.Logf("sameFsTimespec=%v, expected=false", sameFsTimeSpec(ss1.GetLastModification(), ss2.GetLastModification()))
+
+
        // Replace dir with file
        if err := os.RemoveAll(path.Join(root, "dir2")); err != nil {
                t.Fatal(err)