Bonus Token by adjustavailableTokens

[kingsamchen]

I noticed that inside the function adjustavailableTokens()

func (tb *Bucket) adjustavailableTokens(tick int64) {
	if tb.availableTokens >= tb.capacity {
		return
	}
	tb.availableTokens += (tick - tb.latestTick) * tb.quantum
	if tb.availableTokens > tb.capacity {
		tb.availableTokens = tb.capacity
	}
	tb.latestTick = tick
	return
}

the tb.latestTick is not updated if tb.availableTokens >= tb.capacity。

That makes the description of the variable latestTick holds the latest tick for which we know the number of tokens in the bucket. not so accurate, IMO.

And I wrote a snippet of code which can produce surprising results:

func main() {
	bucket := ratelimit.NewBucketWithQuantum(time.Second*1, 100, 20)
	fmt.Printf("Avail=%d\n", bucket.Available())

	fmt.Printf("%v\n", time.Now())
	fmt.Printf("Pause wait for 5s\n")
	time.Sleep(time.Second * 5)
	fmt.Printf("%v\n", time.Now())
	fmt.Printf("Avail=%d\n", bucket.Available())

	fmt.Printf("Request token up to 100\n")
	cnt := bucket.TakeAvailable(100)
	fmt.Printf("Token taken=%d\n", cnt)

        // It will surprise you.
	fmt.Printf("Avail=%d\n", bucket.Available())
}

Output

Avail=100                                                                                          
2019-09-26 01:12:47.9410106 +0800 CST m=+0.003992001                                                Pause wait for 5s                                                                                   
2019-09-26 01:12:52.9614404 +0800 CST m=+5.024421801                                                Avail=100                                                                                           
Request token up to 100                                                                             
Token taken=100                                                                                     
Avail=100             

That is, after taken all tokens out of the bucket, the bucket is still full.

Is this by design or just an implementation bug?

[manadart]

Thanks for this; it does indeed look like a bug. I'll add a fix for it soon.
Alternatively, if you already have one, feel free to propose it.

[kingsamchen]

I proposed a PR to fix this, and your review comments are welcome.

I also added a test case for this issue in case future changes might need a regression test.

FYI: I noticed there are some test-cases failures, they might need some careful examination and fix.

[manadart]

Resolved via #31.