os: adjust error in Stat on windows

Current code could return a non-nil os.FileInfo even if there is an error.
This is a bit incompatible with Stat on other OSes.

Change-Id: I37b608da234f957bb89b82509649de78ccc70bbb
Reviewed-on: https://go-review.googlesource.com/40330
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -174,6 +174,45 @@ func TestStat(t *testing.T) {
 	}
 }
 
+func TestStatError(t *testing.T) {
+	defer chtmpdir(t)()
+
+	path := "no-such-file"
+	Remove(path) // Just in case
+
+	fi, err := Stat(path)
+	if err == nil {
+		t.Fatal("got nil, want error")
+	}
+	if fi != nil {
+		t.Errorf("got %v, want nil", fi)
+	}
+	if perr, ok := err.(*PathError); !ok {
+		t.Errorf("got %T, want %T", err, perr)
+	}
+
+	testenv.MustHaveSymlink(t)
+
+	link := "symlink"
+	Remove(link) // Just in case
+	err = Symlink(path, link)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Remove(link)
+
+	fi, err = Stat(link)
+	if err == nil {
+		t.Fatal("got nil, want error")
+	}
+	if fi != nil {
+		t.Errorf("got %v, want nil", fi)
+	}
+	if perr, ok := err.(*PathError); !ok {
+		t.Errorf("got %T, want %T", err, perr)
+	}
+}
+
 func TestFstat(t *testing.T) {
 	path := sfdir + "/" + sfname
 	file, err1 := Open(path)

src/os/stat_windows.go

@@ -66,14 +66,14 @@ func Stat(name string) (FileInfo, error) {
 	for i := 0; i < 255; i++ {
 		fi, err = Lstat(name)
 		if err != nil {
-			return fi, err
+			return nil, err
 		}
 		if fi.Mode()&ModeSymlink == 0 {
 			return fi, nil
 		}
 		newname, err := Readlink(name)
 		if err != nil {
-			return fi, err
+			return nil, err
 		}
 		if isAbs(newname) {
 			name = newname