os: fix race condition between (*os.File).Stat and os.Chdir on windows

Fixes #13752

Change-Id: I53cfc4ecae90c35b6f1074f3be08489c408a6464
Reviewed-on: https://go-review.googlesource.com/18181Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/os/os_windows_test.go

@@ -150,3 +150,32 @@ func TestBadNetPathError(t *testing.T) {
 		t.Fatal("os.IsNotExist(syscall.Errno(53)) is false, but want true")
 	}
 }
+
+func TestStatDir(t *testing.T) {
+	defer chtmpdir(t)()
+
+	f, err := os.Open(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	fi, err := f.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = os.Chdir("..")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fi2, err := f.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !os.SameFile(fi, fi2) {
+		t.Fatal("race condition occured")
+	}
+}

src/os/stat_windows.go

@@ -20,7 +20,7 @@ func (file *File) Stat() (FileInfo, error) {
 	}
 	if file.isdir() {
 		// I don't know any better way to do that for directory
-		return Stat(file.name)
+		return Stat(file.dirinfo.path)
 	}
 	if file.name == DevNull {
 		return &devNullStat, nil