os: treat "nul" as DevNull file on windows

Also add more tests to test both nul and NUL on windows.

Fixes #24482

Change-Id: I3dfe68ec8de7f90ca869c1096dde0054df3c5cf6
Reviewed-on: https://go-review.googlesource.com/102457
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

3 files changed, 99 insertions, 12 deletions

diff --git a/src/os/os_test.go b/src/os/os_test.go
index 65a2d7e23a..38cbfca272 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -1791,23 +1791,60 @@ func TestSameFile(t *testing.T) {
 	}
 }
 
-func TestDevNullFile(t *testing.T) {
-	f, err := Open(DevNull)
+func testDevNullFileInfo(t *testing.T, statname, devNullName string, fi FileInfo, ignoreCase bool) {
+	pre := fmt.Sprintf("%s(%q): ", statname, devNullName)
+	name := filepath.Base(devNullName)
+	if ignoreCase {
+		if strings.ToUpper(fi.Name()) != strings.ToUpper(name) {
+			t.Errorf(pre+"wrong file name have %v want %v", fi.Name(), name)
+		}
+	} else {
+		if fi.Name() != name {
+			t.Errorf(pre+"wrong file name have %v want %v", fi.Name(), name)
+		}
+	}
+	if fi.Size() != 0 {
+		t.Errorf(pre+"wrong file size have %d want 0", fi.Size())
+	}
+	if fi.Mode()&ModeDevice == 0 {
+		t.Errorf(pre+"wrong file mode %q: ModeDevice is not set", fi.Mode())
+	}
+	if fi.Mode()&ModeCharDevice == 0 {
+		t.Errorf(pre+"wrong file mode %q: ModeCharDevice is not set", fi.Mode())
+	}
+	if fi.Mode().IsRegular() {
+		t.Errorf(pre+"wrong file mode %q: IsRegular returns true", fi.Mode())
+	}
+}
+
+func testDevNullFile(t *testing.T, devNullName string, ignoreCase bool) {
+	f, err := Open(devNullName)
 	if err != nil {
-		t.Fatalf("Open(%s): %v", DevNull, err)
+		t.Fatalf("Open(%s): %v", devNullName, err)
 	}
 	defer f.Close()
+
 	fi, err := f.Stat()
 	if err != nil {
-		t.Fatalf("Stat(%s): %v", DevNull, err)
+		t.Fatalf("Stat(%s): %v", devNullName, err)
 	}
-	name := filepath.Base(DevNull)
-	if fi.Name() != name {
-		t.Fatalf("wrong file name have %v want %v", fi.Name(), name)
+	testDevNullFileInfo(t, "f.Stat", devNullName, fi, ignoreCase)
+
+	fi, err = Stat(devNullName)
+	if err != nil {
+		t.Fatalf("Stat(%s): %v", devNullName, err)
 	}
-	if fi.Size() != 0 {
-		t.Fatalf("wrong file size have %d want 0", fi.Size())
+	testDevNullFileInfo(t, "Stat", devNullName, fi, ignoreCase)
+
+	fi, err = Lstat(devNullName)
+	if err != nil {
+		t.Fatalf("Lstat(%s): %v", devNullName, err)
 	}
+	testDevNullFileInfo(t, "Lstat", devNullName, fi, ignoreCase)
+}
+
+func TestDevNullFile(t *testing.T) {
+	testDevNullFile(t, DevNull, false)
 }
 
 var testLargeWrite = flag.Bool("large_write", false, "run TestLargeWriteToConsole test that floods console with output")
diff --git a/src/os/os_windows_test.go b/src/os/os_windows_test.go
index 12cd9c1f2e..25f93cf131 100644
--- a/src/os/os_windows_test.go
+++ b/src/os/os_windows_test.go
@@ -979,3 +979,35 @@ func TestOneDrive(t *testing.T) {
 	}
 	testIsDir(t, dir, fi)
 }
+
+func TestWindowsDevNullFile(t *testing.T) {
+	testDevNullFile(t, "NUL", true)
+	testDevNullFile(t, "nul", true)
+	testDevNullFile(t, "Nul", true)
+
+	f1, err := os.Open("NUL")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f1.Close()
+
+	fi1, err := f1.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	f2, err := os.Open("nul")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f2.Close()
+
+	fi2, err := f2.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !os.SameFile(fi1, fi2) {
+		t.Errorf(`"NUL" and "nul" are not the same file`)
+	}
+}
diff --git a/src/os/stat_windows.go b/src/os/stat_windows.go
index 0b54a15447..19cc0cf6b7 100644
--- a/src/os/stat_windows.go
+++ b/src/os/stat_windows.go
@@ -8,6 +8,24 @@ import (
 	"syscall"
 )
 
+// isNulName returns true if name is NUL file name.
+// For example, it returns true for both "NUL" and "nul".
+func isNulName(name string) bool {
+	if len(name) != 3 {
+		return false
+	}
+	if name[0] != 'n' && name[0] != 'N' {
+		return false
+	}
+	if name[1] != 'u' && name[1] != 'U' {
+		return false
+	}
+	if name[2] != 'l' && name[2] != 'L' {
+		return false
+	}
+	return true
+}
+
 // Stat returns the FileInfo structure describing file.
 // If there is an error, it will be of type *PathError.
 func (file *File) Stat() (FileInfo, error) {
@@ -19,7 +37,7 @@ func (file *File) Stat() (FileInfo, error) {
 		// I don't know any better way to do that for directory
 		return Stat(file.dirinfo.path)
 	}
-	if file.name == DevNull {
+	if isNulName(file.name) {
 		return &devNullStat, nil
 	}
 
@@ -45,7 +63,7 @@ func statNolog(name string) (FileInfo, error) {
 	if len(name) == 0 {
 		return nil, &PathError{"Stat", name, syscall.Errno(syscall.ERROR_PATH_NOT_FOUND)}
 	}
-	if name == DevNull {
+	if isNulName(name) {
 		return &devNullStat, nil
 	}
 	namep, err := syscall.UTF16PtrFromString(fixLongPath(name))
@@ -80,7 +98,7 @@ func lstatNolog(name string) (FileInfo, error) {
 	if len(name) == 0 {
 		return nil, &PathError{"Lstat", name, syscall.Errno(syscall.ERROR_PATH_NOT_FOUND)}
 	}
-	if name == DevNull {
+	if isNulName(name) {
 		return &devNullStat, nil
 	}
 	namep, err := syscall.UTF16PtrFromString(fixLongPath(name))