Shutdowner interface and Windows trigger (#225)

* Shutdowner interface and Windows trigger

* Updating Shutdown doc

Stop won't be called when Shutdown is.

Co-authored-by: Juan Hernandez <jhernandez@newrelic.com>

service.go

@@ -328,6 +328,16 @@ type Interface interface {
 	Stop(s Service) error
 }
 
+// Shutdowner represents a service interface for a program that differentiates between "stop" and
+// "shutdown". A shutdown is triggered when the whole box (not just the service) is stopped.
+type Shutdowner interface {
+	Interface
+	// Shutdown provides a place to clean up program execution when the system is being shutdown.
+	// It is essentially the same as Stop but for the case where machine is being shutdown/restarted
+	// instead of just normally stopping the service. Stop won't be called when Shutdown is.
+	Shutdown(s Service) error
+}
+
 // TODO: Add Configure to Service interface.
 
 // Service represents a service that can be run or controlled.

service_windows.go

@@ -176,13 +176,26 @@ loop:
 		switch c.Cmd {
 		case svc.Interrogate:
 			changes <- c.CurrentStatus
-		case svc.Stop, svc.Shutdown:
+		case svc.Stop:
 			changes <- svc.Status{State: svc.StopPending}
 			if err := ws.i.Stop(ws); err != nil {
 				ws.setError(err)
 				return true, 2
 			}
 			break loop
+		case svc.Shutdown:
+			changes <- svc.Status{State: svc.StopPending}
+			var err error
+			if wsShutdown, ok := ws.i.(Shutdowner); ok {
+				err = wsShutdown.Shutdown(ws)
+			} else {
+				err = ws.i.Stop(ws)
+			}
+			if err != nil {
+				ws.setError(err)
+				return true, 2
+			}
+			break loop
 		default:
 			continue loop
 		}