os: return some invented data from Stat(DevNull) on windows

Fixes #3321.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5831043

src/pkg/os/os_test.go

@@ -1047,3 +1047,22 @@ func TestSameFile(t *testing.T) {
 		t.Errorf("files should be different")
 	}
 }
+
+func TestDevNullFile(t *testing.T) {
+	f, err := Open(DevNull)
+	if err != nil {
+		t.Fatalf("Open(%s): %v", DevNull, err)
+	}
+	defer f.Close()
+	fi, err := f.Stat()
+	if err != nil {
+		t.Fatalf("Stat(%s): %v", DevNull, err)
+	}
+	name := filepath.Base(DevNull)
+	if fi.Name() != name {
+		t.Fatalf("wrong file name have %v want %v", fi.Name(), name)
+	}
+	if fi.Size() != 0 {
+		t.Fatalf("wrong file size have %d want 0", fi.Size())
+	}
+}

src/pkg/os/stat_windows.go

@@ -21,6 +21,9 @@ func (file *File) Stat() (fi FileInfo, err error) {
 		// I don't know any better way to do that for directory
 		return Stat(file.name)
 	}
+	if file.name == DevNull {
+		return statDevNull()
+	}
 	var d syscall.ByHandleFileInformation
 	e := syscall.GetFileInformationByHandle(syscall.Handle(file.fd), &d)
 	if e != nil {
@@ -41,6 +44,9 @@ func Stat(name string) (fi FileInfo, err error) {
 	if len(name) == 0 {
 		return nil, &PathError{"Stat", name, syscall.Errno(syscall.ERROR_PATH_NOT_FOUND)}
 	}
+	if name == DevNull {
+		return statDevNull()
+	}
 	var d syscall.Win32FileAttributeData
 	e := syscall.GetFileAttributesEx(syscall.StringToUTF16Ptr(name), syscall.GetFileExInfoStandard, (*byte)(unsafe.Pointer(&d)))
 	if e != nil {
@@ -69,6 +75,22 @@ func Lstat(name string) (fi FileInfo, err error) {
 	return Stat(name)
 }
 
+// statDevNull return FileInfo structure describing DevNull file ("NUL").
+// It creates invented data, since none of windows api will return
+// that information.
+func statDevNull() (fi FileInfo, err error) {
+	return &fileStat{
+		name: DevNull,
+		mode: ModeDevice | ModeCharDevice | 0666,
+		sys: &winSys{
+			// hopefully this will work for SameFile
+			vol:   0,
+			idxhi: 0,
+			idxlo: 0,
+		},
+	}, nil
+}
+
 // basename removes trailing slashes and the leading
 // directory name and drive letter from path name.
 func basename(name string) string {