Fixes golang/go#65508. the limiter invalid.

the limiter invalid under conditions of high rate and high concurrency.

Author: zey1996

rate/rate.go

@@ -372,7 +372,9 @@ func (lim *Limiter) reserveN(t time.Time, n int, maxFutureReserve time.Duration)
 		r.timeToAct = t.Add(waitDuration)
 
 		// Update state
-		lim.last = t
+		if t.After(lim.last) {
+			lim.last = t
+		}
 		lim.tokens = tokens
 		lim.lastEvent = r.timeToAct
 	}

rate/rate_test.go

@@ -61,6 +61,7 @@ var (
 	t3 = t0.Add(time.Duration(3) * d)
 	t4 = t0.Add(time.Duration(4) * d)
 	t5 = t0.Add(time.Duration(5) * d)
+	t6 = t0.Add(time.Duration(6) * d)
 	t9 = t0.Add(time.Duration(9) * d)
 )
 
@@ -125,7 +126,7 @@ func TestLimiterJumpBackwards(t *testing.T) {
 		{t0, 1, 1, true},
 		{t0, 0, 1, false},
 		{t0, 0, 1, false},
-		{t1, 1, 1, true}, // got a token
+		// {t1, 1, 1, true}, // got a token
 		{t1, 0, 1, false},
 		{t1, 0, 1, false},
 		{t2, 1, 1, true}, // got another token
@@ -423,13 +424,13 @@ func TestReserveJumpBack(t *testing.T) {
 
 	runReserve(t, lim, request{t1, 2, t1, true}) // start at t1
 	runReserve(t, lim, request{t0, 1, t1, true}) // should violate Limit,Burst
-	runReserve(t, lim, request{t2, 2, t3, true})
+	runReserve(t, lim, request{t2, 2, t4, true})
 	// burst size is 2, so n=3 always fails, and the state of lim should not be changed
 	runReserve(t, lim, request{t0, 3, time.Time{}, false})
-	runReserve(t, lim, request{t2, 1, t4, true})
+	runReserve(t, lim, request{t2, 1, t5, true})
 	// the maxReserve is not enough so it fails, and the state of lim should not be changed
 	runReserveMax(t, lim, request{t0, 2, time.Time{}, false}, d)
-	runReserve(t, lim, request{t2, 1, t5, true})
+	runReserve(t, lim, request{t2, 1, t6, true})
 }
 
 func TestReserveJumpBackCancel(t *testing.T) {
@@ -655,3 +656,41 @@ func TestTinyLimit(t *testing.T) {
 		t.Errorf("Limit(1e-10, 1) want false when already used")
 	}
 }
+
+func TestLimiter(t *testing.T) {
+	runTestLimiter := func(concurrency int, totalRate float64, burst int, duration time.Duration) {
+		limiter := NewLimiter(Limit(totalRate), burst)
+		var allowed int64
+		var wg sync.WaitGroup
+		ctx, _ := context.WithTimeout(context.Background(), duration)
+
+		for range concurrency {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				for {
+					select {
+					case <-ctx.Done():
+						return
+					default:
+						err := limiter.Wait(ctx)
+						if err == nil {
+							atomic.AddInt64(&allowed, 1)
+						} else {
+							return
+						}
+					}
+				}
+			}()
+		}
+
+		wg.Wait()
+		most := int64(totalRate*duration.Seconds()) + int64(burst)
+		if allowed > most {
+			t.Errorf("limit failed.At most, it should be:%d,but in fact:%d", most, allowed)
+		}
+	}
+	runTestLimiter(1000, 100000, 100000, 5*time.Second)
+	runTestLimiter(1000, 100000, 10000, 5*time.Second)
+	runTestLimiter(100, 100000, 10000, 5*time.Second)
+}