discovery: send empty group on empty SD config (#4819)

* discovery: send empty group on blank SD config

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

* Update comments

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

* Add another comment

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

discovery/manager.go

@@ -314,11 +314,13 @@ func (m *Manager) allGroups() map[string][]*targetgroup.Group {
 }
 
 func (m *Manager) registerProviders(cfg sd_config.ServiceDiscoveryConfig, setName string) {
+	var added bool
 	add := func(cfg interface{}, newDiscoverer func() (Discoverer, error)) {
 		t := reflect.TypeOf(cfg).String()
 		for _, p := range m.providers {
 			if reflect.DeepEqual(cfg, p.config) {
 				p.subs = append(p.subs, setName)
+				added = true
 				return
 			}
 		}
@@ -337,6 +339,7 @@ func (m *Manager) registerProviders(cfg sd_config.ServiceDiscoveryConfig, setNam
 			subs:   []string{setName},
 		}
 		m.providers = append(m.providers, &provider)
+		added = true
 	}
 
 	for _, c := range cfg.DNSSDConfigs {
@@ -401,7 +404,17 @@ func (m *Manager) registerProviders(cfg sd_config.ServiceDiscoveryConfig, setNam
 	}
 	if len(cfg.StaticConfigs) > 0 {
 		add(setName, func() (Discoverer, error) {
-			return &StaticProvider{cfg.StaticConfigs}, nil
+			return &StaticProvider{TargetGroups: cfg.StaticConfigs}, nil
+		})
+	}
+	if !added {
+		// Add an empty target group to force the refresh of the corresponding
+		// scrape pool and to notify the receiver that this target set has no
+		// current targets.
+		// It can happen because the combined set of SD configurations is empty
+		// or because we fail to instantiate all the SD configurations.
+		add(setName, func() (Discoverer, error) {
+			return &StaticProvider{TargetGroups: []*targetgroup.Group{&targetgroup.Group{}}}, nil
 		})
 	}
 }

discovery/manager_test.go

@@ -719,6 +719,7 @@ func assertEqualGroups(t *testing.T, got, expected []*targetgroup.Group, msg fun
 }
 
 func verifyPresence(t *testing.T, tSets map[poolKey]map[string]*targetgroup.Group, poolKey poolKey, label string, present bool) {
+	t.Helper()
 	if _, ok := tSets[poolKey]; !ok {
 		t.Fatalf("'%s' should be present in Pool keys: %v", poolKey, tSets)
 		return
@@ -741,7 +742,7 @@ func verifyPresence(t *testing.T, tSets map[poolKey]map[string]*targetgroup.Grou
 		if !present {
 			msg = "not"
 		}
-		t.Fatalf("'%s' should %s be present in Targets labels: %v", label, msg, mergedTargets)
+		t.Fatalf("%q should %s be present in Targets labels: %q", label, msg, mergedTargets)
 	}
 }
 
@@ -781,7 +782,7 @@ scrape_configs:
    - targets: ["foo:9090"]
 `
 	if err := yaml.UnmarshalStrict([]byte(sTwo), cfg); err != nil {
-		t.Fatalf("Unable to load YAML config sOne: %s", err)
+		t.Fatalf("Unable to load YAML config sTwo: %s", err)
 	}
 	c = make(map[string]sd_config.ServiceDiscoveryConfig)
 	for _, v := range cfg.ScrapeConfigs {
@@ -794,6 +795,67 @@ scrape_configs:
 	verifyPresence(t, discoveryManager.targets, poolKey{setName: "prometheus", provider: "string/0"}, "{__address__=\"bar:9090\"}", false)
 }
 
+// TestTargetSetRecreatesEmptyStaticConfigs ensures that reloading a config file after
+// removing all targets from the static_configs sends an update with empty targetGroups.
+// This is required to signal the receiver that this target set has no current targets.
+func TestTargetSetRecreatesEmptyStaticConfigs(t *testing.T) {
+	cfg := &config.Config{}
+
+	sOne := `
+scrape_configs:
+ - job_name: 'prometheus'
+   static_configs:
+   - targets: ["foo:9090"]
+`
+	if err := yaml.UnmarshalStrict([]byte(sOne), cfg); err != nil {
+		t.Fatalf("Unable to load YAML config sOne: %s", err)
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	discoveryManager := NewManager(ctx, log.NewNopLogger())
+	discoveryManager.updatert = 100 * time.Millisecond
+	go discoveryManager.Run()
+
+	c := make(map[string]sd_config.ServiceDiscoveryConfig)
+	for _, v := range cfg.ScrapeConfigs {
+		c[v.JobName] = v.ServiceDiscoveryConfig
+	}
+	discoveryManager.ApplyConfig(c)
+
+	<-discoveryManager.SyncCh()
+	verifyPresence(t, discoveryManager.targets, poolKey{setName: "prometheus", provider: "string/0"}, "{__address__=\"foo:9090\"}", true)
+
+	sTwo := `
+scrape_configs:
+ - job_name: 'prometheus'
+   static_configs:
+`
+	if err := yaml.UnmarshalStrict([]byte(sTwo), cfg); err != nil {
+		t.Fatalf("Unable to load YAML config sTwo: %s", err)
+	}
+	c = make(map[string]sd_config.ServiceDiscoveryConfig)
+	for _, v := range cfg.ScrapeConfigs {
+		c[v.JobName] = v.ServiceDiscoveryConfig
+	}
+	discoveryManager.ApplyConfig(c)
+
+	<-discoveryManager.SyncCh()
+
+	pkey := poolKey{setName: "prometheus", provider: "string/0"}
+	targetGroups, ok := discoveryManager.targets[pkey]
+	if !ok {
+		t.Fatalf("'%v' should be present in target groups", pkey)
+	}
+	group, ok := targetGroups[""]
+	if !ok {
+		t.Fatalf("missing '' key in target groups %v", targetGroups)
+	}
+
+	if len(group.Targets) != 0 {
+		t.Fatalf("Invalid number of targets: expected 0, got %d", len(group.Targets))
+	}
+}
+
 func TestIdenticalConfigurationsAreCoalesced(t *testing.T) {
 	tmpFile, err := ioutil.TempFile("", "sd")
 	if err != nil {