fix backoff manager timer initialization race

staging/src/k8s.io/apimachinery/pkg/util/wait/wait.go

@@ -286,8 +286,9 @@ func contextForChannel(parentCh <-chan struct{}) (context.Context, context.Cance
 }
 
 // BackoffManager manages backoff with a particular scheme based on its underlying implementation. It provides
-// an interface to return a timer for backoff, and caller shall backoff until Timer.C returns. If the second Backoff()
-// is called before the timer from the first Backoff() call finishes, the first timer will NOT be drained.
+// an interface to return a timer for backoff, and caller shall backoff until Timer.C() drains. If the second Backoff()
+// is called before the timer from the first Backoff() call finishes, the first timer will NOT be drained and result in
+// undetermined behavior.
 // The BackoffManager is supposed to be called in a single-threaded environment.
 type BackoffManager interface {
 	Backoff() clock.Timer
@@ -317,7 +318,7 @@ func NewExponentialBackoffManager(initBackoff, maxBackoff, resetDuration time.Du
 			Steps: math.MaxInt32,
 			Cap:   maxBackoff,
 		},
-		backoffTimer:         c.NewTimer(0),
+		backoffTimer:         nil,
 		initialBackoff:       initBackoff,
 		lastBackoffStart:     c.Now(),
 		backoffResetDuration: resetDuration,
@@ -334,9 +335,14 @@ func (b *exponentialBackoffManagerImpl) getNextBackoff() time.Duration {
 	return b.backoff.Step()
 }
 
-// Backoff implements BackoffManager.Backoff, it returns a timer so caller can block on the timer for backoff.
+// Backoff implements BackoffManager.Backoff, it returns a timer so caller can block on the timer for exponential backoff.
+// The returned timer must be drained before calling Backoff() the second time
 func (b *exponentialBackoffManagerImpl) Backoff() clock.Timer {
+	if b.backoffTimer == nil {
+		b.backoffTimer = b.clock.NewTimer(b.getNextBackoff())
+	} else {
 		b.backoffTimer.Reset(b.getNextBackoff())
+	}
 	return b.backoffTimer
 }
 
@@ -354,7 +360,7 @@ func NewJitteredBackoffManager(duration time.Duration, jitter float64, c clock.C
 		clock:        c,
 		duration:     duration,
 		jitter:       jitter,
-		backoffTimer: c.NewTimer(0),
+		backoffTimer: nil,
 	}
 }
 
@@ -366,8 +372,15 @@ func (j *jitteredBackoffManagerImpl) getNextBackoff() time.Duration {
 	return jitteredPeriod
 }
 
+// Backoff implements BackoffManager.Backoff, it returns a timer so caller can block on the timer for jittered backoff.
+// The returned timer must be drained before calling Backoff() the second time
 func (j *jitteredBackoffManagerImpl) Backoff() clock.Timer {
-	j.backoffTimer.Reset(j.getNextBackoff())
+	backoff := j.getNextBackoff()
+	if j.backoffTimer == nil {
+		j.backoffTimer = j.clock.NewTimer(backoff)
+	} else {
+		j.backoffTimer.Reset(backoff)
+	}
 	return j.backoffTimer
 }
 

staging/src/k8s.io/apimachinery/pkg/util/wait/wait_test.go

@@ -731,3 +731,30 @@ func TestJitteredBackoffManagerGetNextBackoff(t *testing.T) {
 		t.Errorf("backoff should be 1, but got %d", backoff)
 	}
 }
+
+func TestJitterBackoffManagerWithRealClock(t *testing.T) {
+	backoffMgr := NewJitteredBackoffManager(1*time.Millisecond, 0, &clock.RealClock{})
+	for i := 0; i < 5; i++ {
+		start := time.Now()
+		<-backoffMgr.Backoff().C()
+		passed := time.Now().Sub(start)
+		if passed < 1*time.Millisecond {
+			t.Errorf("backoff should be at least 1ms, but got %s", passed.String())
+		}
+	}
+}
+
+func TestExponentialBackoffManagerWithRealClock(t *testing.T) {
+	// backoff at least 1ms, 2ms, 4ms, 8ms, 10ms, 10ms, 10ms
+	durationFactors := []time.Duration{1, 2, 4, 8, 10, 10, 10}
+	backoffMgr := NewExponentialBackoffManager(1*time.Millisecond, 10*time.Millisecond, 1*time.Hour, 2.0, 0.0, &clock.RealClock{})
+
+	for i := range durationFactors {
+		start := time.Now()
+		<-backoffMgr.Backoff().C()
+		passed := time.Now().Sub(start)
+		if passed < durationFactors[i]*time.Millisecond {
+			t.Errorf("backoff should be at least %d ms, but got %s", durationFactors[i], passed.String())
+		}
+	}
+}