windows: correct types and error values of internal GUID handling

This corrects the Windows int type to be the more correct int32 Go
analog, as well as not using GetLastError() for the error value of the
GUID string parsing function.

Change-Id: I9716f991ef649f7d299295e3f4e75d3986ec3a74
Reviewed-on: https://go-review.googlesource.com/c/sys/+/181397
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/syscall_windows.go

@@ -288,8 +288,8 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	SetVolumeLabel(rootPathName *uint16, volumeName *uint16) (err error) = SetVolumeLabelW
 //sys	SetVolumeMountPoint(volumeMountPoint *uint16, volumeName *uint16) (err error) = SetVolumeMountPointW
 //sys	MessageBox(hwnd Handle, text *uint16, caption *uint16, boxtype uint32) (ret int32, err error) [failretval==0] = user32.MessageBoxW
-//sys	clsidFromString(lpsz *uint16, pclsid *GUID) (err error) [failretval!=0] = ole32.CLSIDFromString
-//sys	stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int) (chars int) = ole32.StringFromGUID2
+//sys	clsidFromString(lpsz *uint16, pclsid *GUID) (ret error) = ole32.CLSIDFromString
+//sys	stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int32) (chars int32) = ole32.StringFromGUID2
 //sys	coCreateGuid(pguid *GUID) (ret error) = ole32.CoCreateGuid
 
 // syscall interface implementation for other packages
@@ -1276,7 +1276,7 @@ func GenerateGUID() (GUID, error) {
 // in the form of "{XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}".
 func (guid GUID) String() string {
 	var str [100]uint16
-	chars := stringFromGUID2(&guid, &str[0], len(str))
+	chars := stringFromGUID2(&guid, &str[0], int32(len(str)))
 	if chars <= 1 {
 		return ""
 	}

windows/syscall_windows_test.go

@@ -191,4 +191,8 @@ func TestGUID(t *testing.T) {
 	if guid2 != guid {
 		t.Fatalf("Did not parse string back to original GUID = %q; want %q", guid2, guid)
 	}
+	_, err = windows.GUIDFromString("not-a-real-guid")
+	if err != syscall.Errno(windows.CO_E_CLASSSTRING) {
+		t.Fatalf("Bad GUID string error = %v; want CO_E_CLASSSTRING", err)
+	}
 }

windows/zsyscall_windows.go

@@ -2436,21 +2436,17 @@ func MessageBox(hwnd Handle, text *uint16, caption *uint16, boxtype uint32) (ret
 	return
 }
 
-func clsidFromString(lpsz *uint16, pclsid *GUID) (err error) {
-	r1, _, e1 := syscall.Syscall(procCLSIDFromString.Addr(), 2, uintptr(unsafe.Pointer(lpsz)), uintptr(unsafe.Pointer(pclsid)), 0)
-	if r1 != 0 {
-		if e1 != 0 {
-			err = errnoErr(e1)
-		} else {
-			err = syscall.EINVAL
-		}
+func clsidFromString(lpsz *uint16, pclsid *GUID) (ret error) {
+	r0, _, _ := syscall.Syscall(procCLSIDFromString.Addr(), 2, uintptr(unsafe.Pointer(lpsz)), uintptr(unsafe.Pointer(pclsid)), 0)
+	if r0 != 0 {
+		ret = syscall.Errno(r0)
 	}
 	return
 }
 
-func stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int) (chars int) {
+func stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int32) (chars int32) {
 	r0, _, _ := syscall.Syscall(procStringFromGUID2.Addr(), 3, uintptr(unsafe.Pointer(rguid)), uintptr(unsafe.Pointer(lpsz)), uintptr(cchMax))
-	chars = int(r0)
+	chars = int32(r0)
 	return
 }