vmalert: fix blocking hot-reload process if the old rule group hasn't started yet (#7258)

Group
[sleeps](daa7183749/app/vmalert/rule/group.go (L320))
random duration before start the evaluation, and during the sleep,
`g.updateCh <- new` will be blocked since there is no `<-g.updateCh`
waiting.

---------

Signed-off-by: hagen1778 <roman@victoriametrics.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>

app/vmalert/rule/group.go

@@ -324,6 +324,8 @@ func (g *Group) Start(ctx context.Context, nts func() []notifier.Notifier, rw re
 		g.infof("will start in %v", sleepBeforeStart)
 
 		sleepTimer := time.NewTimer(sleepBeforeStart)
+	randSleep:
+		for {
 			select {
 			case <-ctx.Done():
 				sleepTimer.Stop()
@@ -331,7 +333,19 @@ func (g *Group) Start(ctx context.Context, nts func() []notifier.Notifier, rw re
 			case <-g.doneCh:
 				sleepTimer.Stop()
 				return
+			case ng := <-g.updateCh:
+				g.mu.Lock()
+				err := g.updateWith(ng)
+				if err != nil {
+					logger.Errorf("group %q: failed to update: %s", g.Name, err)
+					g.mu.Unlock()
+					continue
+				}
+				g.mu.Unlock()
+				g.infof("reload successfully")
 			case <-sleepTimer.C:
+				break randSleep
+			}
 		}
 		evalTS = evalTS.Add(sleepBeforeStart)
 	}

app/vmalert/rule/group_test.go

@@ -175,6 +175,74 @@ func TestUpdateWith(t *testing.T) {
 	})
 }
 
+func TestUpdateDuringRandSleep(t *testing.T) {
+	// enable rand sleep to test group update during sleep
+	SkipRandSleepOnGroupStart = false
+	defer func() {
+		SkipRandSleepOnGroupStart = true
+	}()
+	rule := AlertingRule{
+		Name: "jobDown",
+		Expr: "up==0",
+		Labels: map[string]string{
+			"foo": "bar",
+		},
+	}
+	g := &Group{
+		Name: "test",
+		Rules: []Rule{
+			&rule,
+		},
+		// big interval ensures big enough randSleep during start process
+		Interval: 100 * time.Hour,
+		updateCh: make(chan *Group),
+	}
+	go g.Start(context.Background(), nil, nil, nil)
+
+	rule1 := AlertingRule{
+		Name: "jobDown",
+		Expr: "up{job=\"vmagent\"}==0",
+		Labels: map[string]string{
+			"foo": "bar",
+		},
+	}
+	g1 := &Group{
+		Rules: []Rule{
+			&rule1,
+		},
+	}
+	g.updateCh <- g1
+	time.Sleep(10 * time.Millisecond)
+	g.mu.RLock()
+	if g.Rules[0].(*AlertingRule).Expr != "up{job=\"vmagent\"}==0" {
+		t.Fatalf("expected to have updated rule expr")
+	}
+	g.mu.RUnlock()
+
+	rule2 := AlertingRule{
+		Name: "jobDown",
+		Expr: "up{job=\"vmagent\"}==0",
+		Labels: map[string]string{
+			"foo": "bar",
+			"baz": "qux",
+		},
+	}
+	g2 := &Group{
+		Rules: []Rule{
+			&rule2,
+		},
+	}
+	g.updateCh <- g2
+	time.Sleep(10 * time.Millisecond)
+	g.mu.RLock()
+	if len(g.Rules[0].(*AlertingRule).Labels) != 2 {
+		t.Fatalf("expected to have updated labels")
+	}
+	g.mu.RUnlock()
+
+	g.Close()
+}
+
 func TestGroupStart(t *testing.T) {
 	const (
 		rules = `

docs/changelog/CHANGELOG.md

@@ -34,6 +34,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent/): support `m` unit for `minutes` duration in command-line flag `-streamAggr.dedupInterval`. Previously unit `m` wasn't supported correctly.
 * BUGFIX: [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): properly apply replication factor when storage node groups are used and replication factor is configured via global value such as `-replicationFactor=2`. Previously, global replication factor was ignored for storage node groups. See [these docs](https://docs.victoriametrics.com/cluster-victoriametrics/#vmstorage-groups-at-vmselect) for more information about storage groups configuration.
 * BUGFIX: `vmselect` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): properly process response in [multi-level cluster setup](https://docs.victoriametrics.com/cluster-victoriametrics/#multi-level-cluster-setup). Before, vmselect could return no data in multi-level setup. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7270) for details. The issue was introduced in [v1.104.0](https://docs.victoriametrics.com/changelog/#v11040).
+* BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert): properly apply configuration changes during hot-reload to rule groups that haven't started yet. Previously, configuration updates to such groups could have resulted into blocking all evaluations within the group, until vmalert restart.
 
 ## [v1.104.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.104.0)