[Elastic Agent] Fix AckBatch to do nothing when no actions passed. (e…

…lastic#25562)

* Fix AckBatch to do nothing when no actions passed.

* Add changelog.

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -62,6 +62,7 @@
 - Don't log when upgrade capability doesn't apply {pull}25386[25386]
 - Fixed issue when unversioned home is set and invoked watcher failing with ENOENT {issue}25371[25371]
 - Fixed Elastic Agent: expecting Dict and received *transpiler.Key for '0' {issue}24453[24453]
+- Fix AckBatch to do nothing when no actions passed {pull}25562[25562]
 
 ==== New features
 

x-pack/elastic-agent/pkg/fleetapi/acker/fleet/fleet_acker.go

@@ -79,6 +79,11 @@ func (f *Acker) AckBatch(ctx context.Context, actions []fleetapi.Action) error {
 		ids = append(ids, action.ID())
 	}
 
+	if len(events) == 0 {
+		// no events to send (nothing to do)
+		return nil
+	}
+
 	cmd := fleetapi.NewAckCmd(f.agentInfo, f.client)
 	req := &fleetapi.AckRequest{
 		Events: events,

x-pack/elastic-agent/pkg/fleetapi/acker/fleet/fleet_acker_test.go

@@ -22,7 +22,7 @@ import (
 	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/fleetapi"
 )
 
-func TestAcker(t *testing.T) {
+func TestAcker_AckCommit(t *testing.T) {
 	type ackRequest struct {
 		Events []fleetapi.AckEvent `json:"events"`
 	}
@@ -69,6 +69,80 @@ func TestAcker(t *testing.T) {
 	}
 }
 
+func TestAcker_AckBatch(t *testing.T) {
+	type ackRequest struct {
+		Events []fleetapi.AckEvent `json:"events"`
+	}
+
+	log, _ := logger.New("fleet_acker", false)
+	client := newTestingClient()
+	agentInfo := &testAgentInfo{}
+	acker, err := NewAcker(log, agentInfo, client)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if acker == nil {
+		t.Fatal("acker not initialized")
+	}
+
+	testID1 := "ack-test-action-id-1"
+	testAction1 := &fleetapi.ActionUnknown{ActionID: testID1}
+	testID2 := "ack-test-action-id-2"
+	testAction2 := &fleetapi.ActionUnknown{ActionID: testID2}
+
+	ch := client.Answer(func(headers http.Header, body io.Reader) (*http.Response, error) {
+		content, err := ioutil.ReadAll(body)
+		assert.NoError(t, err)
+		cr := &ackRequest{}
+		err = json.Unmarshal(content, &cr)
+		assert.NoError(t, err)
+
+		assert.EqualValues(t, 2, len(cr.Events))
+		assert.EqualValues(t, testID1, cr.Events[0].ActionID)
+		assert.EqualValues(t, testID2, cr.Events[1].ActionID)
+
+		resp := wrapStrToResp(http.StatusOK, `{ "actions": [] }`)
+		return resp, nil
+	})
+
+	go func() {
+		for range ch {
+		}
+	}()
+
+	if err := acker.AckBatch(context.Background(), []fleetapi.Action{testAction1, testAction2}); err != nil {
+		t.Fatal(err)
+	}
+	if err := acker.Commit(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestAcker_AckBatch_Empty(t *testing.T) {
+	log, _ := logger.New("fleet_acker", false)
+	client := newNotCalledClient()
+	agentInfo := &testAgentInfo{}
+	acker, err := NewAcker(log, agentInfo, client)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if acker == nil {
+		t.Fatal("acker not initialized")
+	}
+
+	if err := acker.AckBatch(context.Background(), []fleetapi.Action{}); err != nil {
+		t.Fatal(err)
+	}
+	if err := acker.Commit(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	if client.called {
+		t.Fatal("client should not have been used")
+	}
+}
+
 type clientCallbackFunc func(headers http.Header, body io.Reader) (*http.Response, error)
 
 type testingClient struct {
@@ -122,3 +196,30 @@ func wrapStrToResp(code int, body string) *http.Response {
 		Header:        make(http.Header),
 	}
 }
+
+type notCalledClient struct {
+	sync.Mutex
+	called bool
+}
+
+func (t *notCalledClient) Send(
+	_ context.Context,
+	method string,
+	path string,
+	params url.Values,
+	headers http.Header,
+	body io.Reader,
+) (*http.Response, error) {
+	t.Lock()
+	defer t.Unlock()
+	t.called = true
+	return nil, fmt.Errorf("should not have been called")
+}
+
+func (t *notCalledClient) URI() string {
+	return "http://localhost"
+}
+
+func newNotCalledClient() *notCalledClient {
+	return &notCalledClient{}
+}