[Elastic Agent] Improve GRPC stop to be more relaxed. (elastic#20118)

* Improve stop to be more relaxed.

* Add changelog.

(cherry picked from commit 3811728)

x-pack/elastic-agent/CHANGELOG.asciidoc

@@ -52,6 +52,7 @@
 - Remove support for logs type and use logfile {pull}19761[19761]
 - Avoid comparing uncomparable types on enroll {issue}19976[19976]
 - Fix issues with merging of elastic-agent.yml and fleet.yml {pull}20026[20026]
+- Improve GRPC stop to be more relaxed {pull}20118[20118]
 
 ==== New features
 

x-pack/elastic-agent/pkg/core/server/server.go

@@ -526,6 +526,7 @@ func (as *ApplicationState) WriteConnInfo(w io.Writer) error {
 // the application times out during stop and ErrApplication
 func (as *ApplicationState) Stop(timeout time.Duration) error {
 	as.checkinLock.Lock()
+	wasConn := as.checkinDone != nil
 	cfgIdx := as.statusConfigIdx
 	as.expected = proto.StateExpected_STOPPING
 	as.checkinLock.Unlock()
@@ -548,8 +549,10 @@ func (as *ApplicationState) Stop(timeout time.Duration) error {
 		s := as.status
 		doneChan := as.checkinDone
 		as.checkinLock.RUnlock()
-		if s == proto.StateObserved_STOPPING && doneChan == nil {
-			// sent stopping and now is disconnected (so its stopped)
+		if (wasConn && doneChan == nil) || (!wasConn && s == proto.StateObserved_STOPPING && doneChan == nil) {
+			// either occurred
+			// * client was connected then disconnected on stop
+			// * client was not connected; connected; received stopping; then disconnected
 			as.Destroy()
 			return nil
 		}

x-pack/elastic-agent/pkg/core/server/server_test.go

@@ -416,6 +416,57 @@ func TestServer_Stop(t *testing.T) {
 	assert.NoError(t, stopErr)
 }
 
+func TestServer_StopJustDisconnect(t *testing.T) {
+	initConfig := "initial_config"
+	app := &StubApp{}
+	srv := createAndStartServer(t, &StubHandler{})
+	defer srv.Stop()
+	as, err := srv.Register(app, initConfig)
+	require.NoError(t, err)
+	cImpl := &StubClientImpl{}
+	c := newClientFromApplicationState(t, as, cImpl)
+	require.NoError(t, c.Start(context.Background()))
+	defer c.Stop()
+
+	// clients should get initial check-ins then set as healthy
+	require.NoError(t, waitFor(func() error {
+		if cImpl.Config() != initConfig {
+			return fmt.Errorf("client never got intial config")
+		}
+		return nil
+	}))
+	c.Status(proto.StateObserved_HEALTHY, "Running", nil)
+	assert.NoError(t, waitFor(func() error {
+		if app.Status() != proto.StateObserved_HEALTHY {
+			return fmt.Errorf("server never updated currect application state")
+		}
+		return nil
+	}))
+
+	// send stop to the client
+	done := make(chan bool)
+	var stopErr error
+	go func() {
+		stopErr = as.Stop(time.Second * 5)
+		close(done)
+	}()
+
+	// process of testing the flow
+	//   1. server sends stop
+	//   2. client disconnects
+	require.NoError(t, waitFor(func() error {
+		if cImpl.Stop() == 0 {
+			return fmt.Errorf("client never got expected stop")
+		}
+		return nil
+	}))
+	c.Stop()
+	<-done
+
+	// no error on stop
+	assert.NoError(t, stopErr)
+}
+
 func TestServer_StopTimeout(t *testing.T) {
 	initConfig := "initial_config"
 	app := &StubApp{}