Windows: fix service termination (elastic#18916)

Update the Windows service handling logic so that the service doesn't
transition to the STOPPED state until the beater is terminated.

Before this patch, a Beats service would report to be STOPPED as soon
as it received the stop request. This causes some problems during service
restarts, as the new service would start while the old one was still cleaning
up.

Fixes elastic#18914

CHANGELOG.next.asciidoc

@@ -119,6 +119,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix an issue where error messages are not accurate in mapstriface. {issue}18662[18662] {pull}18663[18663]
 - Fix regression in `add_kubernetes_metadata`, so configured `indexers` and `matchers` are used if defaults are not disabled. {issue}18481[18481] {pull}18818[18818]
 - Fix potential race condition in fingerprint processor. {pull}18738[18738]
+- Fixed a service restart failure under Windows. {issue}18914[18914] {pull}18916[18916]
 
 *Auditbeat*
 

libbeat/cmd/instance/beat.go

@@ -389,6 +389,12 @@ func (b *Beat) launch(settings Settings, bt beat.Creator) error {
 		return err
 	}
 
+	// Windows: Mark service as stopped.
+	// After this is run, a Beat service is considered by the OS to be stopped
+	// and another instance of the process can be started.
+	// This must be the first deferred cleanup task (last to execute).
+	defer svc.NotifyTermination()
+
 	// Try to acquire exclusive lock on data path to prevent another beat instance
 	// sharing same data path.
 	bl := newLocker(b)

libbeat/service/service.go

@@ -67,6 +67,11 @@ func HandleSignals(stopFunction func(), cancel context.CancelFunc) {
 	})
 }
 
+// NotifyTermination tells the OS that the service is stopped.
+func NotifyTermination() {
+	notifyWindowsServiceStopped()
+}
+
 // cmdline flags
 var memprofile, cpuprofile, httpprof *string
 var cpuOut *os.File

libbeat/service/service_unix.go

@@ -22,3 +22,6 @@ package service
 // ProcessWindowsControlEvents is not used on non-windows platforms.
 func ProcessWindowsControlEvents(stopCallback func()) {
 }
+
+func notifyWindowsServiceStopped() {
+}

libbeat/service/service_windows.go

@@ -28,7 +28,15 @@ import (
 	"github.com/elastic/beats/v7/libbeat/logp"
 )
 
-type beatService struct{}
+type beatService struct {
+	stopCallback func()
+	done         chan struct{}
+}
+
+var serviceInstance = &beatService{
+	stopCallback: nil,
+	done:         make(chan struct{}, 0),
+}
 
 // Execute runs the beat service with the arguments and manages changes that
 // occur in the environment or runtime that may affect the beat.
@@ -52,9 +60,22 @@ loop:
 		}
 	}
 	changes <- svc.Status{State: svc.StopPending}
+	m.stopCallback()
+	// Block until notifyWindowsServiceStopped below is called. This is required
+	// as the windows/svc package will transition the service to STOPPED state
+	// once this function returns.
+	<-m.done
 	return
 }
 
+func (m *beatService) stop() {
+	close(m.done)
+}
+
+func notifyWindowsServiceStopped() {
+	serviceInstance.stop()
+}
+
 // couldNotConnect is the errno for ERROR_FAILED_SERVICE_CONTROLLER_CONNECT.
 const couldNotConnect syscall.Errno = 1063
 
@@ -76,10 +97,10 @@ func ProcessWindowsControlEvents(stopCallback func()) {
 		run = debug.Run
 	}
 
-	err = run(os.Args[0], &beatService{})
+	serviceInstance.stopCallback = stopCallback
+	err = run(os.Args[0], serviceInstance)
 
 	if err == nil {
-		stopCallback()
 		return
 	}