Notifier: fix deadlock when zero alerts

When all alerts were dropped after alert relabeling, the `sendAll()`
function didn't release the lock properly which created a deadlock with
the Alertmanager target discovery.

In addition, the commit detects early when there are no Alertmanager
endpoint to notify to avoid unnecessary work.

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

notifier/notifier.go

@@ -471,6 +471,10 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 		numSuccess atomic.Uint64
 	)
 	for _, ams := range amSets {
+		if len(ams.ams) == 0 {
+			continue
+		}
+
 		var (
 			payload  []byte
 			err      error
@@ -482,6 +486,7 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 		if len(ams.cfg.AlertRelabelConfigs) > 0 {
 			amAlerts = relabelAlerts(ams.cfg.AlertRelabelConfigs, labels.Labels{}, alerts)
 			if len(amAlerts) == 0 {
+				ams.mtx.RUnlock()
 				continue
 			}
 			// We can't use the cached values from previous iteration.

notifier/notifier_test.go

@@ -219,17 +219,19 @@ func TestHandlerSendAllRemapPerAm(t *testing.T) {
 		errc      = make(chan error, 1)
 		expected1 = make([]*Alert, 0, maxBatchSize)
 		expected2 = make([]*Alert, 0, maxBatchSize)
+		expected3 = make([]*Alert, 0)
 
-		status1, status2 atomic.Int32
+		statusOK atomic.Int32
 	)
-	status1.Store(int32(http.StatusOK))
-	status2.Store(int32(http.StatusOK))
+	statusOK.Store(int32(http.StatusOK))
 
-	server1 := newTestHTTPServerBuilder(&expected1, errc, "", "", &status1)
-	server2 := newTestHTTPServerBuilder(&expected2, errc, "", "", &status2)
+	server1 := newTestHTTPServerBuilder(&expected1, errc, "", "", &statusOK)
+	server2 := newTestHTTPServerBuilder(&expected2, errc, "", "", &statusOK)
+	server3 := newTestHTTPServerBuilder(&expected3, errc, "", "", &statusOK)
 
 	defer server1.Close()
 	defer server2.Close()
+	defer server3.Close()
 
 	h := NewManager(&Options{}, nil)
 	h.alertmanagers = make(map[string]*alertmanagerSet)
@@ -247,38 +249,68 @@ func TestHandlerSendAllRemapPerAm(t *testing.T) {
 		},
 	}
 
-	h.alertmanagers["1"] = &alertmanagerSet{
+	am3Cfg := config.DefaultAlertmanagerConfig
+	am3Cfg.Timeout = model.Duration(time.Second)
+	am3Cfg.AlertRelabelConfigs = []*relabel.Config{
+		{
+			SourceLabels: model.LabelNames{"alertname"},
+			Action:       "drop",
+			Regex:        relabel.MustNewRegexp(".+"),
+		},
+	}
+
+	h.alertmanagers = map[string]*alertmanagerSet{
+		// Drop no alerts.
+		"1": {
 			ams: []alertmanager{
 				alertmanagerMock{
 					urlf: func() string { return server1.URL },
 				},
 			},
 			cfg: &am1Cfg,
-	}
-
-	h.alertmanagers["2"] = &alertmanagerSet{
+		},
+		// Drop only alerts with the "alertnamedrop" label.
+		"2": {
 			ams: []alertmanager{
 				alertmanagerMock{
 					urlf: func() string { return server2.URL },
 				},
 			},
 			cfg: &am2Cfg,
+		},
+		// Drop all alerts.
+		"3": {
+			ams: []alertmanager{
+				alertmanagerMock{
+					urlf: func() string { return server3.URL },
+				},
+			},
+			cfg: &am3Cfg,
+		},
+		// Empty list of Alertmanager endpoints.
+		"4": {
+			ams: []alertmanager{},
+			cfg: &config.DefaultAlertmanagerConfig,
+		},
 	}
 
 	for i := range make([]struct{}, maxBatchSize/2) {
-		h.queue = append(h.queue, &Alert{
+		h.queue = append(h.queue,
+			&Alert{
 				Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
-		})
-		h.queue = append(h.queue, &Alert{
-			Labels: labels.FromStrings("alertnamedrop", fmt.Sprintf("%d", i)),
-		})
+			},
+			&Alert{
+				Labels: labels.FromStrings("alertname", "test", "alertnamedrop", fmt.Sprintf("%d", i)),
+			},
+		)
 
-		expected1 = append(expected1, &Alert{
+		expected1 = append(expected1,
+			&Alert{
 				Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
-		})
-		expected1 = append(expected1, &Alert{
-			Labels: labels.FromStrings("alertnamedrop", fmt.Sprintf("%d", i)),
-		})
+			}, &Alert{
+				Labels: labels.FromStrings("alertname", "test", "alertnamedrop", fmt.Sprintf("%d", i)),
+			},
+		)
 
 		expected2 = append(expected2, &Alert{
 			Labels: labels.FromStrings("alertname", fmt.Sprintf("%d", i)),
@@ -296,6 +328,13 @@ func TestHandlerSendAllRemapPerAm(t *testing.T) {
 
 	require.True(t, h.sendAll(h.queue...), "all sends failed unexpectedly")
 	checkNoErr()
+
+	// Verify that individual locks are released.
+	for k := range h.alertmanagers {
+		h.alertmanagers[k].mtx.Lock()
+		h.alertmanagers[k].ams = nil
+		h.alertmanagers[k].mtx.Unlock()
+	}
 }
 
 func TestCustomDo(t *testing.T) {