Cron scaler: Fix unexpected instance count change

[zou2699]

fix IsActive in time boundary

Signed-off-by: tux zou2699@gmail.com

Provide a description of what has been changed

Checklist

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Fixes #
fix IsActive in time boundary
for example, when we use this cron configuration to scale pod at the last day of a month

start: 0 8 28-31 * *
end: 0 20 1 * *

if runTime is 2022-01-30T7:59:59.9999999
might happen
nextStartTime 2022-01-30T8:00:00.000000
currentTime 2022-01-30T8:00:00.000111
at this time currentTime < nextStartTime will be false，IsActive will return true，trigger unexpected scaler

Fixes: #3854

[JorTurFer]

Hey,
What is the problem you are trying to solve? I mean, even the description is true, doing that change moves this "issue" to the end time instead of starting time. I think that you shouldn't expect millisecond accuracy in the cron scaler, because it's evaluated every X seconds, indeed.
The scenario you described is correct, but the trigger will be activated/deactivated in the next cycle, usually in 30 seconds

[zou2699]

We use the following expression to expand the capacity from the end of 8:00 am on the last day of each month to 8:00 pm on the 1st of the next month

start: "0 8 L * *"
end: "0 20 1 * *"

But the preexisting dependency library cron does not support L (the last day of month), see issue robfig/cron#464

So we use the following expression instead

start:"0 8 28,29,30,31 * *"
end: "0 20 1 * *"

But currently cronScaler.IsActive may trigger an issue like

     // if  execTime=2022.10.30 07:59:59.999
      nextStartTime, startTimecronErr := getCronTime(location, s.metadata.start)
      // got nextStartTime=2022.10.30 08:00:00.000
       ......
      nextEndTime, endTimecronErr := getCronTime(location, s.metadata.end)
     // got nextEndTime=2022.11.01 20:00:00.000
      ......
     currentTime := time.Now().Unix()
     // got currentTime=2022.10.30 08:00:00.111
     switch {
       case nextStartTime < nextEndTime && currentTime < nextStartTime:
            return false, nil
       // will use this case, return true, will scale up pod
      // but in the next cycle,  will retrun false       
       case currentTime <= nextEndTime:
           return true, nil
       default:
           return false, nil
}

this will cause an unexpected scale up event

when we get the currentTime before getting nextStartTime in cronScaler.IsActive, we can avoid this mistake

[zroubalik]

@zou2699 could you please open an issue first, with clear explanation of the problem? Thanks!

[zroubalik]

@zou2699 do you see any usecases (that don't suffer from this edge case) where this fix could break the existing behavior?

[zou2699]

@zroubalik This will not affect other usecases

[JorTurFer]

I think that the change doesn't change the behavior, more than changing the edge case of 59:999. It's true that the opposite case (00.001) will change, but as I said, I think that it's a really "forced" edge case and the change will be done in the next polling instead of current (30 secs later) and we are assuming that this evaluated exactly at 59.999 which is so complicated

[JorTurFer]

BTW @zou2699 , are you sure that the problem is related with this? I mean, what pooling interval do you have? Because if you are not checking ever 1 second (and even in that case), I'd say that it's really complex to happen

[zou2699]

The generation of nextStartTime depends on the current time point, but the currentTime here is obtained after nextStartTime is generated, which will lead to the following problems

For example, in the above example, we don't want an unexpected instance count change at 2022.10.30 07:59:59.999, we only expect to instance count change at 2022.10.31 08:00:00.000 -> 2022.11.01 20:00:00.000 .

Through this adjustment, the nextStartTime of the time that is not the last day (October 28, October 29, October 30) is always greater than the currentTime:

case1:
currentTime: 2022-10-28 07:59:59.999
nextStartTime: 2022-10-28 08:00:00.000
case2:
currentTime: 2022-10-28 08:00:00.001
nextStartTime: 2022-10-29 08:00:00.000

to avoid unexpected instance count changes, thus supporting 'L' in Cron expressions

[zroubalik]

@zou2699 could you please fix the static check problems and add entry to the changelog, we can merge it then.

[zroubalik]

/run-e2e cron*
Update: You can check the progress here