windows: replace all StringToUTF16 calls with UTF16FromString

`StringToUTF16` is deprecated and will panic if given an "invalid"
string (in particular, one that has a null byte in it).  The replacement
function is `UTF16FromString`, and it returns an error if there was
a problem.

This change replaces all uses of `StringToUTF16` with `UTF16FromString`.
The `service` struct now no longer stores a `string` name but rather
a `*uint16` pointer to the name.

It should not be possible to panic due to UTF16 string conversion
at this point.

Fixes golang/go#73006

Change-Id: Idce9cdbb4651fef8481f0cad19b5df0314fd4277
Reviewed-on: https://go-review.googlesource.com/c/sys/+/659936
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

windows/svc/service.go

@@ -132,10 +132,10 @@ type ctlEvent struct {
 
 // service provides access to windows service api.
 type service struct {
-	name    string
-	h       windows.Handle
-	c       chan ctlEvent
-	handler Handler
+	namePointer *uint16
+	h           windows.Handle
+	c           chan ctlEvent
+	handler     Handler
 }
 
 type exitCode struct {
@@ -209,7 +209,7 @@ var theService service // This is, unfortunately, a global, which means only one
 // serviceMain is the entry point called by the service manager, registered earlier by
 // the call to StartServiceCtrlDispatcher.
 func serviceMain(argc uint32, argv **uint16) uintptr {
-	handle, err := windows.RegisterServiceCtrlHandlerEx(windows.StringToUTF16Ptr(theService.name), ctlHandlerCallback, 0)
+	handle, err := windows.RegisterServiceCtrlHandlerEx(theService.namePointer, ctlHandlerCallback, 0)
 	if sysErr, ok := err.(windows.Errno); ok {
 		return uintptr(sysErr)
 	} else if err != nil {
@@ -280,15 +280,21 @@ loop:
 
 // Run executes service name by calling appropriate handler function.
 func Run(name string, handler Handler) error {
+	// Check to make sure that the service name is valid.
+	namePointer, err := windows.UTF16PtrFromString(name)
+	if err != nil {
+		return err
+	}
+
 	initCallbacks.Do(func() {
 		ctlHandlerCallback = windows.NewCallback(ctlHandler)
 		serviceMainCallback = windows.NewCallback(serviceMain)
 	})
-	theService.name = name
+	theService.namePointer = namePointer
 	theService.handler = handler
 	theService.c = make(chan ctlEvent)
 	t := []windows.SERVICE_TABLE_ENTRY{
-		{ServiceName: windows.StringToUTF16Ptr(theService.name), ServiceProc: serviceMainCallback},
+		{ServiceName: namePointer, ServiceProc: serviceMainCallback},
 		{ServiceName: nil, ServiceProc: 0},
 	}
 	return windows.StartServiceCtrlDispatcher(&t[0])