Add period for rate limiter middleware

pkg/middlewares/ratelimiter/rate_limiter.go

@@ -29,7 +29,7 @@ type rateLimiter struct {
 	rate  rate.Limit // reqs/s
 	burst int64
 	// maxDelay is the maximum duration we're willing to wait for a bucket reservation to become effective, in nanoseconds.
-	// For now it is somewhat arbitrarily set to 1/rate.
+	// For now it is somewhat arbitrarily set to 1/(2*rate).
 	maxDelay      time.Duration
 	sourceMatcher utils.SourceExtractor
 	next          http.Handler
@@ -61,20 +61,34 @@ func New(ctx context.Context, next http.Handler, config dynamic.RateLimit, name
 	}
 
 	burst := config.Burst
-	if burst <= 0 {
+	if burst < 1 {
 		burst = 1
 	}
 
-	// Logically, we should set maxDelay to ~infinity when config.Average == 0 (because it means to rate limiting),
+	period := time.Duration(config.Period)
+	if period == 0 {
+		period = time.Second
+	}
+
+	// Logically, we should set maxDelay to infinity when config.Average == 0 (because it means no rate limiting),
 	// but since the reservation will give us a delay = 0 anyway in this case, we're good even with any maxDelay >= 0.
 	var maxDelay time.Duration
-	if config.Average != 0 {
-		maxDelay = time.Second / time.Duration(config.Average*2)
+	var rtl float64
+	if config.Average > 0 {
+		rtl = float64(config.Average*int64(time.Second)) / float64(period)
+		// maxDelay does not scale well for rates below 1,
+		// so we just cap it to the corresponding value, i.e. 0.5s, in order to keep the effective rate predictable.
+		// One alternative would be to switch to a no-reservation mode (Allow() method) whenever we are in such a low rate regime.
+		if rtl < 1 {
+			maxDelay = 500 * time.Millisecond
+		} else {
+			maxDelay = time.Second / (time.Duration(rtl) * 2)
+		}
 	}
 
 	return &rateLimiter{
 		name:          name,
-		rate:          rate.Limit(config.Average),
+		rate:          rate.Limit(rtl),
 		burst:         burst,
 		maxDelay:      maxDelay,
 		next:          next,