fs: fix setting timestamps on broken symlinks

Use UtimesNanoAt with AT_SYMLINK_NOFOLLOW. Add
PosixTest/SetattrSymlink to test this.

Change-Id: Ia3a00ad75afd1ba255965025f63fd047662885ec

fs/loopback.go

@@ -403,20 +403,23 @@ func (n *LoopbackNode) Setattr(ctx context.Context, f FileHandle, in *fuse.SetAt
 		atime, aok := in.GetATime()
 
 		if mok || aok {
-
-			ap := &atime
-			mp := &mtime
-			if !aok {
-				ap = nil
+			ta := unix.Timespec{Nsec: unix.UTIME_OMIT}
+			tm := unix.Timespec{Nsec: unix.UTIME_OMIT}
+			var err error
+			if aok {
+				ta, err = unix.TimeToTimespec(atime)
+				if err != nil {
+					return ToErrno(err)
+				}
 			}
-			if !mok {
-				mp = nil
+			if mok {
+				tm, err = unix.TimeToTimespec(mtime)
+				if err != nil {
+					return ToErrno(err)
+				}
 			}
-			var ts [2]syscall.Timespec
-			ts[0] = fuse.UtimeToTimespec(ap)
-			ts[1] = fuse.UtimeToTimespec(mp)
-
-			if err := syscall.UtimesNano(p, ts[:]); err != nil {
+			ts := []unix.Timespec{ta, tm}
+			if err := unix.UtimesNanoAt(unix.AT_FDCWD, p, ts, unix.AT_SYMLINK_NOFOLLOW); err != nil {
 				return ToErrno(err)
 			}
 		}

posixtest/test.go

@@ -48,6 +48,32 @@ var All = map[string]func(*testing.T, string){
 	"DirSeek":                    DirSeek,
 	"FcntlFlockSetLk":            FcntlFlockSetLk,
 	"FcntlFlockLocksFile":        FcntlFlockLocksFile,
+	"SetattrSymlink":             SetattrSymlink,
+}
+
+func SetattrSymlink(t *testing.T, mnt string) {
+	l := filepath.Join(mnt, "link")
+	if err := os.Symlink("doesnotexist", l); err != nil {
+		t.Fatalf("symlink: %v", err)
+	}
+
+	tvs := []unix.Timeval{
+		{Sec: 42, Usec: 1},
+		{Sec: 43, Usec: 2},
+	}
+	if err := unix.Lutimes(l, tvs); err != nil {
+		t.Fatalf("Lutimes: %v", err)
+	}
+
+	var st unix.Stat_t
+	if err := unix.Lstat(l, &st); err != nil {
+		t.Fatalf("Lstat: %v", err)
+	}
+
+	if st.Mtim.Sec != 43 {
+		// Can't check atime; it's hard to prevent implicit readlink calls.
+		t.Fatalf("got mtime %v, want 43", st.Mtim)
+	}
 }
 
 func DirectIO(t *testing.T, mnt string) {