fakeTicker doesn't work as expected

[Yongkan Huang]

Hi,

	fc := clockwork.NewFakeClock()

	ft := fc.NewTicker(1)
	fc.Advance(3)

	select {
	case x := <-ft.Chan():
		fmt.Println("1-------------", x)
	default:
	}

I am expecting have 3 print in above code, but it only print once, looks like the ticker is only triggered once, how can I trigger multiple times?

[Darren Jacques]

There is no loop in the code above. It will only ever print once.

Also keep in mind ticks that are not received before the follow on tick are dropped: "The ticker will adjust the time interval or drop ticks to make up for slow receivers" - https://pkg.go.dev/time#Ticker

To accomplish what you want without race conditions, you need 3 goroutines running waiting on the tick, then call Advance(3). You should see each once of them fire once.

[Yongkan Huang]

func main() {
	fc := clockwork.NewFakeClock()
	ft := fc.NewTicker(1)

	for i := 0; i < 3; i++ {
		go func() {
			select {
			case x := <-ft.Chan():
				fmt.Println("1-------------", x)
			default:
				fmt.Println("2-------------")
			}
		}()
	}

	fc.Advance(3)
	time.Sleep(time.Second)
}

-----OUTPUT----
2-------------
1------------- 2006-01-02 15:04:05.000000001 +0000 UTC
2-------------

[Darren Jacques]

The default: case in your switch conditions leads you to race conditions. You are asking the program to run through the for loop as fast as possible, sidestepping the channel read if it can, so that is what happens. You just happen to catch 1 of the ticks in the code you have above.

• If you remove the default: case, you'll receive all the ticks, although not necessarily in order, since again, they all race.
• If you add a Println to ticker.go, you'll see it send ticks in order, as expected.
• If you want to avoid race conditions on the caller side, you need to advance across each tick, take the action you want to take, then advance again. Otherwise clockwork will run as fast as possible, which is quite fast.

From clockwork's perspective, this is all working as intended. The caller side code is setting up a race condition.

[Yongkan Huang]

@DPJacques

Thanks for your reply!
it always print once even I removed the default case, it prints twice occasionally sometime.
I did a time.Sleep at the end to leave ticker enough time to get triggered.

[Darren Jacques]

This is because the go routines have not started by the time fc.Advance(3) has completed, once again there is a race condition. If you add a time.Sleep(1) before your fc.Advance(3) you should see most of them.

However, you are still inducing race conditions on the clockwork.Ticker interface. Even with the time.Sleep, you are only delaying for an arbitrary amount of time in hopes that the goroutines are all setup and blocking by the time you call fc.Advance(3). Pragmatically, this is almost always true, but niether Go or clockwork can't guarantee that, because it depends on the runtime scheduler.

The root cause is that clockwork.Ticker can't guarantee to send all ticks while also remaining consistent with the time.Ticker API, becuase this is not a guarantee the time.Ticker API makes.

If you want to avoid race conditions, you need to advance past every tick individually. If you don't do this, there will always be some amount of race condition in your code. There are ways to minimize those race conditions, but they will still be present. In all cases, clockwork is working as intended.

@sagikazarmark I think it would be reasonable to close this issue out as working as intended.

[Darren Jacques]

Example of what I refer to in my comment above, the following code prints nothing:

func main() {
	fc := clockwork.NewFakeClock()
	ft := fc.NewTicker(1)

	for i := 0; i < 3; i++ {
		i := i
		go func() {
			fmt.Println("started: ", i)
			select {
			case x := <-ft.Chan():
				fmt.Println(i, " time: ", x)
			}
		}()
	}

	fc.Advance(3)
}

[Darren Jacques]

Another example:

func main() {
	fc := clockwork.NewFakeClock()
	ft := fc.NewTicker(1)

	for i := 0; i < 3; i++ {
		i := i
		go func() {
			fmt.Println("started: ", i)
			select {
			case x := <-ft.Chan():
				fmt.Println(i, " time: ", x)
			}
		}()
	}

	fc.Advance(3)
	fmt.Println("advanced")
	time.Sleep(time.Second)
}

Output:

advanced
started:  2
2  time:  2006-01-02 15:04:05.000000001 +0000 UTC
started:  1
started:  0

[Márk Sági-Kazár]

@sagikazarmark I think it would be reasonable to close this issue out as working as intended.

I tend to agree.

@seaguest a couple more comments that hopefully understand you what's happening:

• similarly how the real Ticker works in the time package, the underlying channel in the fake ticker is a buffered channel with a buffer of one. That means if a channel is "full", the rest of the ticks will get lost.
• due to that fact and the race condition @DPJacques pointed out, you may not get all of the ticks

@DPJacques's examples accurately represent all that.

If you still believe there is a bug somewhere, please provide a test case or an example that proves there is one.

Hope this helps!