windows/svc: add Context to ChangeRequest

New Context field will be used in the following CL to test
ctlHandler parameter alignments.

Also adjust TestExample to pass hard coded Context value of 123456
to test service, and verify that correct value is logged. Final
part of the test is commented out, and will be adjusted in the next
CL.

Updates golang/go#25660

Change-Id: Iad2896ae497ee1edc0d62655eaf08671ec2651c5
Reviewed-on: https://go-review.googlesource.com/c/158697
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/svc/service.go

@@ -80,6 +80,7 @@ type ChangeRequest struct {
 	EventType     uint32
 	EventData     uintptr
 	CurrentStatus Status
+	Context       uintptr
 }
 
 // Handler is the interface that must be implemented to build Windows service.
@@ -121,12 +122,11 @@ func init() {
 	cRegisterServiceCtrlHandlerExW = a.MustFindProc("RegisterServiceCtrlHandlerExW").Addr()
 }
 
-// The HandlerEx prototype also has a context pointer but since we don't use
-// it at start-up time we don't have to pass it over either.
 type ctlEvent struct {
 	cmd       Cmd
 	eventType uint32
 	eventData uintptr
+	context   uintptr
 	errno     uint32
 }
 
@@ -238,13 +238,12 @@ func (s *service) run() {
 		exitFromHandler <- exitCode{ss, errno}
 	}()
 
-	status := Status{State: Stopped}
 	ec := exitCode{isSvcSpecific: true, errno: 0}
+	outcr := ChangeRequest{
+		CurrentStatus: Status{State: Stopped},
+	}
 	var outch chan ChangeRequest
 	inch := s.c
-	var cmd Cmd
-	var evtype uint32
-	var evdata uintptr
 loop:
 	for {
 		select {
@@ -255,10 +254,11 @@ loop:
 			}
 			inch = nil
 			outch = cmdsToHandler
-			cmd = r.cmd
-			evtype = r.eventType
-			evdata = r.eventData
-		case outch <- ChangeRequest{cmd, evtype, evdata, status}:
+			outcr.Cmd = r.cmd
+			outcr.EventType = r.eventType
+			outcr.EventData = r.eventData
+			outcr.Context = r.context
+		case outch <- outcr:
 			inch = s.c
 			outch = nil
 		case c := <-changesFromHandler:
@@ -271,7 +271,7 @@ loop:
 				}
 				break loop
 			}
-			status = c
+			outcr.CurrentStatus = c
 		case ec = <-exitFromHandler:
 			break loop
 		}
@@ -316,7 +316,7 @@ func Run(name string, handler Handler) error {
 	}
 
 	ctlHandler := func(ctl uint32, evtype uint32, evdata uintptr, context uintptr) uintptr {
-		e := ctlEvent{cmd: Cmd(ctl), eventType: evtype, eventData: evdata}
+		e := ctlEvent{cmd: Cmd(ctl), eventType: evtype, eventData: evdata, context: context}
 		// We assume that this callback function is running on
 		// the same thread as Run. Nowhere in MS documentation
 		// I could find statement to guarantee that. So putting