Allow the result map for the series set before hand with a hint.

Signed-off-by: gotjosh <josue.abreu@gmail.com>

rules/alerting.go

@@ -542,6 +542,13 @@ func (r *AlertingRule) ForEachActiveAlert(f func(*Alert)) {
 	}
 }
 
+func (r *AlertingRule) ActiveAlertsCount() int {
+	r.activeMtx.Lock()
+	defer r.activeMtx.Unlock()
+
+	return len(r.active)
+}
+
 func (r *AlertingRule) sendAlerts(ctx context.Context, ts time.Time, resendDelay, interval time.Duration, notifyFunc NotifyFunc) {
 	alerts := []*Alert{}
 	r.ForEachActiveAlert(func(alert *Alert) {

rules/alerting_test.go

@@ -1027,3 +1027,22 @@ func TestAlertingRule_SetNoDependencyRules(t *testing.T) {
 	rule.SetNoDependencyRules(true)
 	require.True(t, rule.NoDependencyRules())
 }
+
+func TestAlertingRule_ActiveAlertsCount(t *testing.T) {
+	rule := NewAlertingRule(
+		"TestRule",
+		nil,
+		time.Minute,
+		0,
+		labels.FromStrings("severity", "critical"),
+		labels.EmptyLabels(), labels.EmptyLabels(), "", true, nil,
+	)
+
+	// Set an active alert.
+	lbls := labels.FromStrings("a1", "1")
+	h := lbls.Hash()
+	al := &Alert{State: StateFiring, Labels: lbls, ActiveAt: time.Now()}
+	rule.active[h] = al
+
+	require.Equal(t, 1, t rule.ActiveAlertsCount())
+}

rules/group.go

@@ -681,7 +681,8 @@ func (g *Group) RestoreForState(ts time.Time) {
 			continue
 		}
 
-		seriesByLabels := map[string]storage.Series{}
+		// While not technically the same number of series we expect, it's as good of an approximation as any.
+		seriesByLabels := make(map[string]storage.Series, alertRule.ActiveAlertsCount())
 		for sset.Next() {
 			seriesByLabels[sset.At().Labels().DropMetricName().String()] = sset.At()
 		}