os: return an error when the argument of Mkdir on Windows is os.DevNull

Test added.

Fixes golang#24556

Change-Id: I4d1cd4513142edeea1a983fbfde46c2fccecab2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/186139
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/os/file.go

@@ -228,6 +228,9 @@ func (f *File) WriteString(s string) (n int, err error) {
 // bits (before umask).
 // If there is an error, it will be of type *PathError.
 func Mkdir(name string, perm FileMode) error {
+	if runtime.GOOS == "windows" && isWindowsNulName(name) {
+		return &PathError{"mkdir", name, syscall.ENOTDIR}
+	}
 	e := syscall.Mkdir(fixLongPath(name), syscallMode(perm))
 
 	if e != nil {
@@ -560,3 +563,21 @@ func (f *File) SyscallConn() (syscall.RawConn, error) {
 	}
 	return newRawConn(f)
 }
+
+// isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
+// True is returned if name is 'NUL' whatever the case.
+func isWindowsNulName(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
+}

src/os/os_windows_test.go

@@ -1168,3 +1168,19 @@ func TestWindowsReadlink(t *testing.T) {
 	mklink(t, "relfilelink", "file")
 	testReadlink(t, "relfilelink", "file")
 }
+
+// os.Mkdir(os.DevNull) fails.
+func TestMkdirDevNull(t *testing.T) {
+	err := os.Mkdir(os.DevNull, 777)
+	oserr, ok := err.(*os.PathError)
+	if !ok {
+		t.Fatalf("error (%T) is not *os.PathError", err)
+	}
+	errno, ok := oserr.Err.(syscall.Errno)
+	if !ok {
+		t.Fatalf("error (%T) is not syscall.Errno", oserr)
+	}
+	if errno != syscall.ENOTDIR {
+		t.Fatalf("error %d is not syscall.ENOTDIR", errno)
+	}
+}

src/os/stat_windows.go

@@ -10,24 +10,6 @@ import (
 	"unsafe"
 )
 
-// isNulName reports whether 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) {
@@ -39,7 +21,7 @@ func (file *File) Stat() (FileInfo, error) {
 		// I don't know any better way to do that for directory
 		return Stat(file.dirinfo.path)
 	}
-	if isNulName(file.name) {
+	if isWindowsNulName(file.name) {
 		return &devNullStat, nil
 	}
 
@@ -65,7 +47,7 @@ func stat(funcname, name string, createFileAttrs uint32) (FileInfo, error) {
 	if len(name) == 0 {
 		return nil, &PathError{funcname, name, syscall.Errno(syscall.ERROR_PATH_NOT_FOUND)}
 	}
-	if isNulName(name) {
+	if isWindowsNulName(name) {
 		return &devNullStat, nil
 	}
 	namep, err := syscall.UTF16PtrFromString(fixLongPath(name))