Fix race conditions in the memory alerts store (#3648)

* Fix race conditions in the memory alerts store

Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

* Expose the GC method from store.Alerts

Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

* Use RLock/Unlock on read path

Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

* Resolve conflicts

Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

* release locks by using the defer

Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

* Revert the RWMutex back to Mutex

Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

---------

Signed-off-by: Xiaochao Dong (@damnever) <the.xcdong@gmail.com>

provider/mem/mem.go

@@ -34,10 +34,11 @@ const alertChannelLength = 200
 type Alerts struct {
 	cancel context.CancelFunc
 
+	mtx sync.Mutex
+
 	alerts *store.Alerts
 	marker types.AlertMarker
 
-	mtx       sync.Mutex
 	listeners map[int]listeningAlerts
 	next      int
 
@@ -100,37 +101,53 @@ func NewAlerts(ctx context.Context, m types.AlertMarker, intervalGC time.Duratio
 		logger:    log.With(l, "component", "provider"),
 		callback:  alertCallback,
 	}
-	a.alerts.SetGCCallback(func(alerts []types.Alert) {
-		for _, alert := range alerts {
-			// As we don't persist alerts, we no longer consider them after
-			// they are resolved. Alerts waiting for resolved notifications are
-			// held in memory in aggregation groups redundantly.
-			m.Delete(alert.Fingerprint())
-			a.callback.PostDelete(&alert)
-		}
-
-		a.mtx.Lock()
-		for i, l := range a.listeners {
-			select {
-			case <-l.done:
-				delete(a.listeners, i)
-				close(l.alerts)
-			default:
-				// listener is not closed yet, hence proceed.
-			}
-		}
-		a.mtx.Unlock()
-	})
 
 	if r != nil {
 		a.registerMetrics(r)
 	}
 
-	go a.alerts.Run(ctx, intervalGC)
+	go a.gcLoop(ctx, intervalGC)
 
 	return a, nil
 }
 
+func (a *Alerts) gcLoop(ctx context.Context, interval time.Duration) {
+	t := time.NewTicker(interval)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			a.gc()
+		}
+	}
+}
+
+func (a *Alerts) gc() {
+	a.mtx.Lock()
+	defer a.mtx.Unlock()
+
+	deleted := a.alerts.GC()
+	for _, alert := range deleted {
+		// As we don't persist alerts, we no longer consider them after
+		// they are resolved. Alerts waiting for resolved notifications are
+		// held in memory in aggregation groups redundantly.
+		a.marker.Delete(alert.Fingerprint())
+		a.callback.PostDelete(&alert)
+	}
+
+	for i, l := range a.listeners {
+		select {
+		case <-l.done:
+			delete(a.listeners, i)
+			close(l.alerts)
+		default:
+			// listener is not closed yet, hence proceed.
+		}
+	}
+}
+
 // Close the alert provider.
 func (a *Alerts) Close() {
 	if a.cancel != nil {
@@ -174,11 +191,13 @@ func (a *Alerts) GetPending() provider.AlertIterator {
 		ch   = make(chan *types.Alert, alertChannelLength)
 		done = make(chan struct{})
 	)
+	a.mtx.Lock()
+	defer a.mtx.Unlock()
+	alerts := a.alerts.List()
 
 	go func() {
 		defer close(ch)
-
-		for _, a := range a.alerts.List() {
+		for _, a := range alerts {
 			select {
 			case ch <- a:
 			case <-done:
@@ -192,11 +211,16 @@ func (a *Alerts) GetPending() provider.AlertIterator {
 
 // Get returns the alert for a given fingerprint.
 func (a *Alerts) Get(fp model.Fingerprint) (*types.Alert, error) {
+	a.mtx.Lock()
+	defer a.mtx.Unlock()
 	return a.alerts.Get(fp)
 }
 
 // Put adds the given alert to the set.
 func (a *Alerts) Put(alerts ...*types.Alert) error {
+	a.mtx.Lock()
+	defer a.mtx.Unlock()
+
 	for _, alert := range alerts {
 		fp := alert.Fingerprint()
 
@@ -226,14 +250,12 @@ func (a *Alerts) Put(alerts ...*types.Alert) error {
 
 		a.callback.PostStore(alert, existing)
 
-		a.mtx.Lock()
 		for _, l := range a.listeners {
 			select {
 			case l.alerts <- alert:
 			case <-l.done:
 			}
 		}
-		a.mtx.Unlock()
 	}
 
 	return nil
@@ -241,6 +263,9 @@ func (a *Alerts) Put(alerts ...*types.Alert) error {
 
 // count returns the number of non-resolved alerts we currently have stored filtered by the provided state.
 func (a *Alerts) count(state types.AlertState) int {
+	a.mtx.Lock()
+	defer a.mtx.Unlock()
+
 	var count int
 	for _, alert := range a.alerts.List() {
 		if alert.Resolved() {

provider/mem/mem_test.go

@@ -15,6 +15,7 @@ package mem
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"reflect"
 	"strconv"
@@ -561,3 +562,62 @@ func (l *limitCountCallback) PostStore(_ *types.Alert, existing bool) {
 func (l *limitCountCallback) PostDelete(_ *types.Alert) {
 	l.alerts.Dec()
 }
+
+func TestAlertsConcurrently(t *testing.T) {
+	callback := &limitCountCallback{limit: 100}
+	a, err := NewAlerts(context.Background(), types.NewMarker(prometheus.NewRegistry()), time.Millisecond, callback, log.NewNopLogger(), nil)
+	require.NoError(t, err)
+
+	stopc := make(chan struct{})
+	failc := make(chan struct{})
+	go func() {
+		time.Sleep(2 * time.Second)
+		close(stopc)
+	}()
+	expire := 10 * time.Millisecond
+	wg := sync.WaitGroup{}
+	for i := 0; i < 100; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+
+			j := 0
+			for {
+				select {
+				case <-failc:
+					return
+				case <-stopc:
+					return
+				default:
+				}
+				now := time.Now()
+				err := a.Put(&types.Alert{
+					Alert: model.Alert{
+						Labels:   model.LabelSet{"bar": model.LabelValue(strconv.Itoa(j))},
+						StartsAt: now,
+						EndsAt:   now.Add(expire),
+					},
+					UpdatedAt: now,
+				})
+				if err != nil && !errors.Is(err, errTooManyAlerts) {
+					close(failc)
+					return
+				}
+				j++
+			}
+		}()
+	}
+	wg.Wait()
+	select {
+	case <-failc:
+		t.Fatalf("unexpected error happened")
+	default:
+	}
+
+	time.Sleep(expire)
+	require.Eventually(t, func() bool {
+		// When the alert will eventually expire and is considered resolved - it won't count.
+		return a.count(types.AlertStateActive) == 0
+	}, 2*expire, expire)
+	require.Equal(t, int32(0), callback.alerts.Load())
+}

store/store.go

@@ -64,12 +64,13 @@ func (a *Alerts) Run(ctx context.Context, interval time.Duration) {
 		case <-ctx.Done():
 			return
 		case <-t.C:
-			a.gc()
+			a.GC()
 		}
 	}
 }
 
-func (a *Alerts) gc() {
+// GC deletes resolved alerts and returns them.
+func (a *Alerts) GC() []types.Alert {
 	a.Lock()
 	var resolved []types.Alert
 	for fp, alert := range a.c {
@@ -90,6 +91,7 @@ func (a *Alerts) gc() {
 	}
 	a.Unlock()
 	a.cb(resolved)
+	return resolved
 }
 
 // Get returns the Alert with the matching fingerprint, or an error if it is