os: prevent infinite symlink loop of Stat on Windows

The Windows version of Stat calls Readlink iteratively until
reaching a non-symlink file.
If the given file is a circular symlink, It never stops.
This CL defines the maximum number of symlink loop count.
If the loop count will exceed that number, Stat will return error.

Fixes #16538

Change-Id: Ia9f3f2259a8d32801461c5041cc24a34f9f81009
Reviewed-on: https://go-review.googlesource.com/27580
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/os/os_test.go

@@ -1812,3 +1812,26 @@ func TestRemoveAllRace(t *testing.T) {
 	close(hold) // let workers race to remove root
 	wg.Wait()
 }
+
+func TestStatSymlinkLoop(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	defer chtmpdir(t)()
+
+	err := Symlink("x", "y")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Remove("y")
+
+	err = Symlink("y", "x")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Remove("x")
+
+	_, err = Stat("x")
+	if perr, ok := err.(*PathError); !ok || perr.Err != syscall.ELOOP {
+		t.Errorf("expected *PathError with ELOOP, got %T: %v\n", err, err)
+	}
+}

src/os/stat_windows.go

@@ -61,7 +61,7 @@ func (file *File) Stat() (FileInfo, error) {
 func Stat(name string) (FileInfo, error) {
 	var fi FileInfo
 	var err error
-	for {
+	for i := 0; i < 255; i++ {
 		fi, err = Lstat(name)
 		if err != nil {
 			return fi, err
@@ -74,6 +74,7 @@ func Stat(name string) (FileInfo, error) {
 			return fi, err
 		}
 	}
+	return nil, &PathError{"Stat", name, syscall.ELOOP}
 }
 
 // Lstat returns the FileInfo structure describing the named file.