notifier: attach external labels before relabelling

By attaching external labels before relabelling, they are available
during relabelling.

notifier/notifier.go

@@ -211,6 +211,15 @@ func (n *Notifier) Send(alerts ...*model.Alert) {
 	n.mtx.Lock()
 	defer n.mtx.Unlock()
 
+	// Attach external labels before relabelling and sending.
+	for _, a := range alerts {
+		for ln, lv := range n.opts.ExternalLabels {
+			if _, ok := a.Labels[ln]; !ok {
+				a.Labels[ln] = lv
+			}
+		}
+	}
+
 	alerts = n.relabelAlerts(alerts)
 
 	// Queue capacity should be significantly larger than a single alert
@@ -267,15 +276,6 @@ func postURL(u string) string {
 func (n *Notifier) sendAll(alerts ...*model.Alert) int {
 	begin := time.Now()
 
-	// Attach external labels before sending alerts.
-	for _, a := range alerts {
-		for ln, lv := range n.opts.ExternalLabels {
-			if _, ok := a.Labels[ln]; !ok {
-				a.Labels[ln] = lv
-			}
-		}
-	}
-
 	b, err := json.Marshal(alerts)
 	if err != nil {
 		log.Errorf("Encoding alerts failed: %s", err)

notifier/notifier_test.go

@@ -153,7 +153,6 @@ func TestHandlerSendAll(t *testing.T) {
 	h := New(&Options{
 		AlertmanagerURLs: []string{server1.URL, server2.URL},
 		Timeout:          time.Minute,
-		ExternalLabels:   model.LabelSet{"a": "b"},
 	})
 
 	for i := range make([]struct{}, maxBatchSize) {
@@ -165,7 +164,6 @@ func TestHandlerSendAll(t *testing.T) {
 		expected = append(expected, &model.Alert{
 			Labels: model.LabelSet{
 				"alertname": model.LabelValue(fmt.Sprintf("%d", i)),
-				"a":         "b",
 			},
 		})
 	}
@@ -187,6 +185,56 @@ func TestHandlerSendAll(t *testing.T) {
 	}
 }
 
+func TestExternalLabels(t *testing.T) {
+	h := New(&Options{
+		QueueCapacity:  3 * maxBatchSize,
+		ExternalLabels: model.LabelSet{"a": "b"},
+		RelabelConfigs: []*config.RelabelConfig{
+			{
+				SourceLabels: model.LabelNames{"alertname"},
+				TargetLabel:  "a",
+				Action:       "replace",
+				Regex:        config.MustNewRegexp("externalrelabelthis"),
+				Replacement:  "c",
+			},
+		},
+	})
+
+	// This alert should get the external label attached.
+	h.Send(&model.Alert{
+		Labels: model.LabelSet{
+			"alertname": "test",
+		},
+	})
+
+	// This alert should get the external label attached, but then set to "c"
+	// through relabelling.
+	h.Send(&model.Alert{
+		Labels: model.LabelSet{
+			"alertname": "externalrelabelthis",
+		},
+	})
+
+	expected := []*model.Alert{
+		{
+			Labels: model.LabelSet{
+				"alertname": "test",
+				"a":         "b",
+			},
+		},
+		{
+			Labels: model.LabelSet{
+				"alertname": "externalrelabelthis",
+				"a":         "c",
+			},
+		},
+	}
+
+	if !alertsEqual(expected, h.queue) {
+		t.Errorf("Expected alerts %v, got %v", expected, h.queue)
+	}
+}
+
 func TestHandlerRelabel(t *testing.T) {
 	h := New(&Options{
 		QueueCapacity: 3 * maxBatchSize,