RetryAfter returns unexpected values about redis_rate

[vmihailenco]

Hi,

This looks like a bug in Lua script - https://github.com/go-redis/redis_rate/blob/v9/lua.go . It is not immediately clear what is wrong there...

[Bo0mer]

Hi,

I've did this change:

diff --git a/lua.go b/lua.go
index 00a1177..dc6c1f7 100644
--- a/lua.go
+++ b/lua.go
@@ -43,7 +43,7 @@ local new_tat = tat + increment
 local allow_at = new_tat - burst_offset

 local diff = now - allow_at
-local remaining = math.floor(diff / emission_interval + 0.5)
+local remaining = math.floor(diff / emission_interval)

 if remaining < 0 then
   local reset_after = tat - now

And now it seems to report fine. However, I'm not sure why this +0.5 was there, so it may brake some other stuff.

2020/11/12 12:02:24 -1ns
2020/11/12 12:02:24 9.799548998s
2020/11/12 12:02:25 9.598224997s
2020/11/12 12:02:25 9.397761002s
2020/11/12 12:02:25 9.194830998s
2020/11/12 12:02:25 8.991724997s
2020/11/12 12:02:25 8.791387006s
2020/11/12 12:02:26 8.590687006s
2020/11/12 12:02:26 8.388626009s
2020/11/12 12:02:26 8.187648996s
2020/11/12 12:02:26 7.984241008s
2020/11/12 12:02:26 7.779868006s
2020/11/12 12:02:27 7.579449996s
2020/11/12 12:02:27 7.379165008s
2020/11/12 12:02:27 7.178017005s
2020/11/12 12:02:27 6.976421996s
2020/11/12 12:02:27 6.775869995s
2020/11/12 12:02:28 6.573429003s
2020/11/12 12:02:28 6.372973009s
2020/11/12 12:02:28 6.171716004s
2020/11/12 12:02:28 5.971336007s
2020/11/12 12:02:28 5.769253998s
2020/11/12 12:02:29 5.568884s
2020/11/12 12:02:29 5.367034003s
2020/11/12 12:02:29 5.166488006s
2020/11/12 12:02:29 4.966213002s
2020/11/12 12:02:29 4.765294s
2020/11/12 12:02:30 4.565043002s
2020/11/12 12:02:30 4.364315003s
2020/11/12 12:02:30 4.163448005s
2020/11/12 12:02:30 3.962646007s
2020/11/12 12:02:30 3.761032998s
2020/11/12 12:02:31 3.560769006s
2020/11/12 12:02:31 3.357135996s
2020/11/12 12:02:31 3.156644999s
2020/11/12 12:02:31 2.954966008s
2020/11/12 12:02:31 2.753356009s
2020/11/12 12:02:32 2.549777001s
2020/11/12 12:02:32 2.349068999s
2020/11/12 12:02:32 2.148274004s
2020/11/12 12:02:32 1.946217s
2020/11/12 12:02:32 1.744691997s
2020/11/12 12:02:33 1.544281005s
2020/11/12 12:02:33 1.340514004s
2020/11/12 12:02:33 1.140128999s
2020/11/12 12:02:33 939.062997ms
2020/11/12 12:02:33 738.867998ms
2020/11/12 12:02:34 538.006007ms
2020/11/12 12:02:34 337.228ms
2020/11/12 12:02:34 136.642009ms
Last allowed since: 10.065357881
2020/11/12 12:02:34 -1ns
2020/11/12 12:02:34 9.795255005s
2020/11/12 12:02:35 9.593696996s
2020/11/12 12:02:35 9.389268994s
2020/11/12 12:02:35 9.187741994s
2020/11/12 12:02:35 8.983551993s
2020/11/12 12:02:35 8.782241001s
2020/11/12 12:02:36 8.581724002s
2020/11/12 12:02:36 8.377003997s
2020/11/12 12:02:36 8.172646s
2020/11/12 12:02:36 7.971110999s
2020/11/12 12:02:36 7.768710002s
2020/11/12 12:02:37 7.565398991s
2020/11/12 12:02:37 7.365051999s
2020/11/12 12:02:37 7.162248998s
2020/11/12 12:02:37 6.958739995s
2020/11/12 12:02:37 6.757247999s
2020/11/12 12:02:38 6.555720001s
2020/11/12 12:02:38 6.352163001s
2020/11/12 12:02:38 6.151698991s
2020/11/12 12:02:38 5.946638002s
2020/11/12 12:02:38 5.745154991s
2020/11/12 12:02:39 5.543387994s
2020/11/12 12:02:39 5.341874003s
2020/11/12 12:02:39 5.140425994s
2020/11/12 12:02:39 4.940015003s
2020/11/12 12:02:39 4.739174991s
2020/11/12 12:02:40 4.538375005s
2020/11/12 12:02:40 4.333557993s
2020/11/12 12:02:40 4.132116004s
2020/11/12 12:02:40 3.929347991s
2020/11/12 12:02:40 3.727843001s
2020/11/12 12:02:41 3.525530993s
2020/11/12 12:02:41 3.321722999s
2020/11/12 12:02:41 3.120489001s
2020/11/12 12:02:41 2.918787002s
2020/11/12 12:02:41 2.718363001s
2020/11/12 12:02:42 2.517773002s
2020/11/12 12:02:42 2.315688997s
2020/11/12 12:02:42 2.115105003s
2020/11/12 12:02:42 1.914551004s
2020/11/12 12:02:42 1.714140996s
2020/11/12 12:02:43 1.513627991s
2020/11/12 12:02:43 1.310731992s
2020/11/12 12:02:43 1.108968004s
2020/11/12 12:02:43 907.801002ms
2020/11/12 12:02:43 704.115003ms
2020/11/12 12:02:44 501.938998ms
2020/11/12 12:02:44 297.913998ms
2020/11/12 12:02:44 95.117002ms
Last allowed since: 10.109563657
2020/11/12 12:02:44 -1ns
2020/11/12 12:02:44 9.797523006s

[vmihailenco]

Good find!

I'm not sure why this +0.5 was there

Me neither...

so it may brake some other stuff.

Can you come up with a test case? It should be fine to change this code as long as we can make sure it is tested.

[Bo0mer]

@vmihailenco, what is your opinion on the test - is it fine to use time.Sleep or I should implement some polling mechanism?

This one fails with the current lua script implementation but succeeds when the rounding is removed.
I don't like the time.Sleeps, though.

func TestAllow_Remaining(t *testing.T) {
	ctx := context.Background()

	l := rateLimiter()
	limit := redis_rate.Limit{
		Rate:   1,
		Period: 100 * time.Millisecond,
		Burst:  1,
	}

	res, err := l.Allow(ctx, "test_id", limit)
	assert.Nil(t, err)
	assert.Equal(t, res.Allowed, 1)
	assert.Equal(t, res.Remaining, 0)
	assert.Equal(t, res.RetryAfter, time.Duration(-1))
	assert.InDelta(t, res.ResetAfter, 100*time.Millisecond, float64(10*time.Millisecond))

	res, err = l.Allow(ctx, "test_id", limit)
	assert.Nil(t, err)
	assert.Equal(t, res.Allowed, 0)
	assert.Equal(t, res.Remaining, 0)
	assert.InDelta(t, res.RetryAfter, 100*time.Millisecond, float64(10*time.Millisecond))
	assert.InDelta(t, res.ResetAfter, 100*time.Millisecond, float64(10*time.Millisecond))

	// Wait for 2/3 of the RetryAfter time to pass.
	time.Sleep((res.RetryAfter * 2) / 3)
	res, err = l.Allow(ctx, "test_id", limit)
	assert.Equal(t, res.Allowed, 0)

	time.Sleep(res.RetryAfter * 3)
	res, err = l.Allow(ctx, "test_id", limit)
	assert.Equal(t, res.Allowed, 1)
	assert.Equal(t, res.RetryAfter, time.Duration(-1))
}

[vmihailenco]

Thanks! This should be fixed by 19dc8f9 and 78e4497