Fix event monitor panic.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/server/events.go

@@ -17,6 +17,7 @@ limitations under the License.
 package server
 
 import (
+	"sync"
 	"time"
 
 	eventtypes "github.com/containerd/containerd/api/events"
@@ -56,7 +57,9 @@ type eventMonitor struct {
 }
 
 type backOff struct {
-	queuePool     map[string]*backOffQueue
+	queuePool map[string]*backOffQueue
+	// tickerMu is mutex used to protect the ticker.
+	tickerMu      sync.Mutex
 	ticker        *time.Ticker
 	minDuration   time.Duration
 	maxDuration   time.Duration
@@ -120,8 +123,8 @@ func (em *eventMonitor) start() (<-chan struct{}, error) {
 		return nil, errors.New("event channel is nil")
 	}
 	closeCh := make(chan struct{})
+	backOffCheckCh := em.backOff.start()
 	go func() {
-		backOffCheckCh := em.backOff.start()
 		for {
 			select {
 			case e := <-em.ch:
@@ -366,12 +369,18 @@ func (b *backOff) reBackOff(key string, events []interface{}, oldDuration time.D
 }
 
 func (b *backOff) start() <-chan time.Time {
+	b.tickerMu.Lock()
+	defer b.tickerMu.Unlock()
 	b.ticker = time.NewTicker(b.checkDuration)
 	return b.ticker.C
 }
 
 func (b *backOff) stop() {
-	b.ticker.Stop()
+	b.tickerMu.Lock()
+	defer b.tickerMu.Unlock()
+	if b.ticker != nil {
+		b.ticker.Stop()
+	}
 }
 
 func newBackOffQueue(events []interface{}, init time.Duration, c clock.Clock) *backOffQueue {