open immediately if error rate equals to 0

Signed-off-by: Ryan Leung <rleungx@gmail.com>

client/circuitbreaker/circuit_breaker.go

@@ -121,6 +121,7 @@ func (cb *CircuitBreaker[T]) ChangeSettings(apply func(config *Settings)) {
 	defer cb.mutex.Unlock()
 
 	apply(cb.config)
+	log.Info("circuit breaker settings changed", zap.Any("config", cb.config))
 }
 
 // Execute calls the given function if the CircuitBreaker is closed and returns the result of execution.
@@ -251,6 +252,10 @@ func (s *State[T]) onRequest(cb *CircuitBreaker[T]) (*State[T], error) {
 		// continue in closed state till ErrorRateWindow is over
 		return s, nil
 	case StateOpen:
+		if s.cb.config.ErrorRateThresholdPct == 0 {
+			return cb.newState(now, StateClosed), nil
+		}
+
 		if now.After(s.end) {
 			// CoolDownInterval is over, it is time to transition to half-open state
 			log.Info("circuit breaker cooldown period is over. Transitioning to half-open state to test the service",
@@ -262,7 +267,10 @@ func (s *State[T]) onRequest(cb *CircuitBreaker[T]) (*State[T], error) {
 			return s, errs.ErrCircuitBreakerOpen
 		}
 	case StateHalfOpen:
-		fmt.Println("StateHalfOpen", s.failureCount, s.successCount, s.pendingCount, s.cb.config.HalfOpenSuccessCount)
+		if s.cb.config.ErrorRateThresholdPct == 0 {
+			return cb.newState(now, StateClosed), nil
+		}
+
 		// do we need some expire time here in case of one of pending requests is stuck forever?
 		if s.failureCount > 0 {
 			// there were some failures during half-open state, let's go back to open state to wait a bit longer

tests/integrations/client/client_test.go

@@ -2101,3 +2101,51 @@ func TestCircuitBreaker(t *testing.T) {
 		re.NotNil(region)
 	}
 }
+
+func TestCircuitBreakerChangeSettings(t *testing.T) {
+	re := require.New(t)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	cluster, err := tests.NewTestCluster(ctx, 1)
+	re.NoError(err)
+	defer cluster.Destroy()
+
+	circuitBreakerSettings := cb.Settings{
+		ErrorRateThresholdPct: 60,
+		MinQPSForOpen:         10,
+		ErrorRateWindow:       time.Millisecond,
+		CoolDownInterval:      time.Second,
+		HalfOpenSuccessCount:  1,
+	}
+
+	endpoints := runServer(re, cluster)
+	cli := setupCli(ctx, re, endpoints, opt.WithRegionMetaCircuitBreaker(circuitBreakerSettings))
+	defer cli.Close()
+
+	for i := 0; i < 10; i++ {
+		region, err := cli.GetRegion(context.TODO(), []byte("a"))
+		re.NoError(err)
+		re.NotNil(region)
+	}
+
+	re.NoError(failpoint.Enable("github.com/tikv/pd/client/triggerCircuitBreaker", "return(true)"))
+
+	for i := 0; i < 100; i++ {
+		_, err := cli.GetRegion(context.TODO(), []byte("a"))
+		re.Error(err)
+	}
+
+	_, err = cli.GetRegion(context.TODO(), []byte("a"))
+	re.Error(err)
+	re.Contains(err.Error(), "circuit breaker is open")
+
+	cli.UpdateOption(opt.RegionMetadataCircuitBreakerSettings, func(config *cb.Settings) {
+		*config = cb.AlwaysClosedSettings
+	})
+
+	_, err = cli.GetRegion(context.TODO(), []byte("a"))
+	re.Error(err)
+	re.Contains(err.Error(), "ResourceExhausted")
+	re.NoError(failpoint.Disable("github.com/tikv/pd/client/triggerCircuitBreaker"))
+}