fs/diff: support symlink to abspath with second-precision mtime

Previously, `Changes()` for a symlink to absolute path, with second-precision
mtime of the link itself always resulted in ENOENT.

This is because `compareFileContent()` was called for the link target path, without changing the root path.

Fix moby/buildkit#172

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

fs/fstest/file_unix.go

@@ -2,7 +2,13 @@
 
 package fstest
 
-import "github.com/containerd/continuity/sysx"
+import (
+	"path/filepath"
+	"time"
+
+	"github.com/containerd/continuity/sysx"
+	"golang.org/x/sys/unix"
+)
 
 // SetXAttr sets the xatter for the file
 func SetXAttr(name, key, value string) Applier {
@@ -10,3 +16,14 @@ func SetXAttr(name, key, value string) Applier {
 		return sysx.LSetxattr(name, key, []byte(value), 0)
 	})
 }
+
+// Lchtimes changes access and mod time of file without following symlink
+func Lchtimes(name string, atime, mtime time.Time) Applier {
+	return applyFn(func(root string) error {
+		path := filepath.Join(root, name)
+		at := unix.NsecToTimespec(atime.UnixNano())
+		mt := unix.NsecToTimespec(mtime.UnixNano())
+		utimes := [2]unix.Timespec{at, mt}
+		return unix.UtimesNanoAt(unix.AT_FDCWD, path, utimes[0:], unix.AT_SYMLINK_NOFOLLOW)
+	})
+}