Remove ManagerV2 unnecessary reload when apm tracing config is nil

CHANGELOG.next.asciidoc

@@ -115,6 +115,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Ensure Elasticsearch output can always recover from network errors {pull}40794[40794]
 - Add `translate_ldap_attribute` processor. {pull}41472[41472]
 - Remove unnecessary debug logs during idle connection teardown {issue}40824[40824]
+- Remove unnecessary reload for Elastic Agent managed beats when apm tracing config changes from nil to nil {pull}41794[41794]
 - Fix incorrect cloud provider identification in add_cloud_metadata processor using provider priority mechanism {pull}41636[41636]
 
 *Auditbeat*

x-pack/libbeat/management/managerV2.go

@@ -893,6 +893,13 @@ func (cm *BeatV2Manager) reloadAPM(unit *agentUnit) {
 			apmConfig = expected.APMConfig
 		}
 	}
+
+	if (cm.lastAPMCfg == nil && apmConfig == nil) || (cm.lastAPMCfg != nil && gproto.Equal(cm.lastAPMCfg, apmConfig)) {
+		// configuration for the APM tracing did not change; do nothing
+		cm.logger.Debug("Skipped reloading APM tracing; configuration didn't change")
+		return
+	}
+
 	if apmConfig == nil {
 		// APM tracing is being stopped
 		cm.logger.Debug("Stopping APM tracing")
@@ -907,12 +914,6 @@ func (cm *BeatV2Manager) reloadAPM(unit *agentUnit) {
 		return
 	}
 
-	if cm.lastAPMCfg != nil && gproto.Equal(cm.lastAPMCfg, apmConfig) {
-		// configuration for the APM tracing did not change; do nothing
-		cm.logger.Debug("Skipped reloading APM tracing; configuration didn't change")
-		return
-	}
-
 	uconfig, err := conf.NewConfigFrom(apmConfig)
 	if err != nil {
 		cm.logger.Errorf("Failed to create uconfig from APM configuration: %s", err)

x-pack/libbeat/management/managerV2_test.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"github.com/joeshaw/multierror"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/zap/zapcore"
 	"google.golang.org/grpc"
@@ -266,6 +267,141 @@ func TestManagerV2(t *testing.T) {
 	}, 15*time.Second, 300*time.Millisecond)
 }
 
+func TestManagerV2_ReloadCount(t *testing.T) {
+	r := reload.NewRegistry()
+
+	output := &reloadable{}
+	r.MustRegisterOutput(output)
+	inputs := &reloadableList{}
+	r.MustRegisterInput(inputs)
+	apm := &reloadable{}
+	r.MustRegisterAPM(apm)
+
+	inputConfigUpdated := make(chan struct{})
+	onObserved := func(observed *proto.CheckinObserved, currentIdx int) {
+		if currentIdx == 1 {
+			period, err := inputs.Configs()[0].Config.String("period", -1)
+			require.NoError(t, err)
+			if period == "10m" {
+				select {
+				case <-inputConfigUpdated:
+				default:
+					close(inputConfigUpdated)
+				}
+			}
+		}
+	}
+
+	agentInfo := &proto.AgentInfo{
+		Id:       "elastic-agent-id",
+		Version:  version.GetDefaultVersion(),
+		Snapshot: true,
+	}
+	srv := integration.NewMockServer([]*proto.CheckinExpected{
+		{
+			AgentInfo: agentInfo,
+			Units: []*proto.UnitExpected{
+				{
+					Id:             "output-unit",
+					Type:           proto.UnitType_OUTPUT,
+					ConfigStateIdx: 1,
+					Config: &proto.UnitExpectedConfig{
+						Id:   "default",
+						Type: "elasticsearch",
+						Name: "elasticsearch",
+					},
+					State:    proto.State_HEALTHY,
+					LogLevel: proto.UnitLogLevel_INFO,
+				},
+				{
+					Id:             "input-unit-1",
+					Type:           proto.UnitType_INPUT,
+					ConfigStateIdx: 1,
+					Config: &proto.UnitExpectedConfig{
+						Id:   "system/metrics-system-default-system-1",
+						Type: "system/metrics",
+						Name: "system-1",
+						Streams: []*proto.Stream{
+							{
+								Id: "system/metrics-system.filesystem-default-system-1",
+								Source: integration.RequireNewStruct(t, map[string]interface{}{
+									"metricsets": []interface{}{"filesystem"},
+									"period":     "1m",
+								}),
+							},
+						},
+					},
+					State:    proto.State_HEALTHY,
+					LogLevel: proto.UnitLogLevel_INFO,
+				},
+			},
+			Features:    nil,
+			FeaturesIdx: 1,
+		},
+		{
+			AgentInfo: agentInfo,
+			Units: []*proto.UnitExpected{
+				{
+					Id:             "output-unit",
+					Type:           proto.UnitType_OUTPUT,
+					ConfigStateIdx: 1,
+					State:          proto.State_HEALTHY,
+					LogLevel:       proto.UnitLogLevel_INFO,
+				},
+				{
+					Id:             "input-unit-1",
+					Type:           proto.UnitType_INPUT,
+					ConfigStateIdx: 2,
+					Config: &proto.UnitExpectedConfig{
+						Id:   "system/metrics-system-default-system-1",
+						Type: "system/metrics",
+						Name: "system-1",
+						Streams: []*proto.Stream{
+							{
+								Id: "system/metrics-system.filesystem-default-system-1",
+								Source: integration.RequireNewStruct(t, map[string]interface{}{
+									"metricsets": []interface{}{"filesystem"},
+									"period":     "10m",
+								}),
+							},
+						},
+					},
+					State:    proto.State_HEALTHY,
+					LogLevel: proto.UnitLogLevel_INFO,
+				},
+			},
+			Features:    nil,
+			FeaturesIdx: 1,
+		},
+	},
+		onObserved,
+		500*time.Millisecond,
+	)
+	require.NoError(t, srv.Start())
+	defer srv.Stop()
+
+	client := client.NewV2(fmt.Sprintf(":%d", srv.Port), "", client.VersionInfo{
+		Name: "program",
+		Meta: map[string]string{
+			"key": "value",
+		},
+	}, client.WithGRPCDialOptions(grpc.WithTransportCredentials(insecure.NewCredentials())))
+
+	m, err := NewV2AgentManagerWithClient(&Config{
+		Enabled: true,
+	}, r, client)
+	require.NoError(t, err)
+
+	err = m.Start()
+	require.NoError(t, err)
+	defer m.Stop()
+
+	<-inputConfigUpdated
+	assert.Equal(t, 1, output.reloadCount) // initial load
+	assert.Equal(t, 2, inputs.reloadCount) // initial load + config update
+	assert.Equal(t, 0, apm.reloadCount)    // no apm tracing config applied
+}
+
 func TestOutputError(t *testing.T) {
 	// Uncomment the line below to see the debug logs for this test
 	// logp.DevelopmentSetup(logp.WithLevel(logp.DebugLevel), logp.WithSelectors("*"))
@@ -553,19 +689,22 @@ func TestErrorPerUnit(t *testing.T) {
 }
 
 type reloadable struct {
-	mx     sync.Mutex
-	config *reload.ConfigWithMeta
+	mx          sync.Mutex
+	config      *reload.ConfigWithMeta
+	reloadCount int
 }
 
 type reloadableList struct {
-	mx      sync.Mutex
-	configs []*reload.ConfigWithMeta
+	mx          sync.Mutex
+	configs     []*reload.ConfigWithMeta
+	reloadCount int
 }
 
 func (r *reloadable) Reload(config *reload.ConfigWithMeta) error {
 	r.mx.Lock()
 	defer r.mx.Unlock()
 	r.config = config
+	r.reloadCount++
 	return nil
 }
 
@@ -579,6 +718,7 @@ func (r *reloadableList) Reload(configs []*reload.ConfigWithMeta) error {
 	r.mx.Lock()
 	defer r.mx.Unlock()
 	r.configs = configs
+	r.reloadCount++
 	return nil
 }