os: handle relative symlinks correctly in Stat on windows

Walk relative symlinks in windows os.Stat from
symlink path instead of from current directory.

Fixes #19870

Change-Id: I0a27473d11485f073084b1f19b30c5b3a2fbc0f7
Reviewed-on: https://go-review.googlesource.com/39932Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/os_test.go

@@ -1705,6 +1705,43 @@ func TestStatStdin(t *testing.T) {
 	}
 }
 
+func TestStatRelativeSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	tmpdir, err := ioutil.TempDir("", "TestStatRelativeSymlink")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(tmpdir)
+
+	target := filepath.Join(tmpdir, "target")
+	f, err := Create(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	st, err := f.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	link := filepath.Join(tmpdir, "link")
+	err = Symlink(filepath.Base(target), link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	st1, err := Stat(link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !SameFile(st, st1) {
+		t.Error("Stat doesn't follow relative symlink")
+	}
+}
+
 func TestReadAtEOF(t *testing.T) {
 	f := newFile("TestReadAtEOF", t)
 	defer Remove(f.Name())

src/os/stat_windows.go

@@ -71,10 +71,15 @@ func Stat(name string) (FileInfo, error) {
 		if fi.Mode()&ModeSymlink == 0 {
 			return fi, nil
 		}
-		name, err = Readlink(name)
+		newname, err := Readlink(name)
 		if err != nil {
 			return fi, err
 		}
+		if isAbs(newname) {
+			name = newname
+		} else {
+			name = dirname(name) + `\` + newname
+		}
 	}
 	return nil, &PathError{"Stat", name, syscall.ELOOP}
 }