promql: Fix limiting of extrapolation to negative values

[beorn7]

This is a bit tough to explain, but I'll try:

rate & friends have a sophisticated extrapolation algorithm. Usually, we extrapolate the result to the total interval specified in the range selector. However, if the first sample within the range is too far away from the beginning of the interval, or if the last sample within the range is too far away from the end of the interval, we assume the series has just started half a sampling interval before the first sample or after the last sample, respectively, and shorten the extrapolation interval correspondingly. We calculate the sampling interval by looking at the average time between samples within the range, and we define "too far away" as "more than 110% of that sampling interval".

However, if this algorithm leads to an extrapolated starting value that is negative, we limit the start of the extrapolation interval to the point where the extrapolated starting value is zero.

At least that was the intention.

What we actually implemented is the following: If extrapolating all the way to the beginning of the total interval would lead to an extrapolated negative value, we would only extrapolate to the zero point as above, even if the algorithm above would have selected a starting point that is just half a sampling interval before the first sample and that starting point would not have an extrapolated negative value. In other word: What was meant as a limitation of the extrapolation interval yielded a longer extrapolation interval in this case.

There is an exception to the case just described: If the increase of the extrapolation interval is more than 110% of the sampling interval, we suddenly drop back to only extrapolate to half a sampling interval.

This behavior can be nicely seen in the testcounter_zero_cutoff test, where the rate goes up all the way to 0.7 and then jumps back to 0.6.

This commit changes the behavior to what was (presumably) intended from the beginning: The extension of the extrapolation interval is only limited if actually needed to prevent extrapolation to negative values, but the "limitation" never leads to more extrapolation anymore.

The difference is subtle, and probably it never bothered anyone. However, if you calculate a rate of a classic histograms, the old behavior might create non-monotonic histograms as a result (because of the jumps you can see nicely in the old version of the testcounter_zero_cutoff test). With this fix, that doesn't happen anymore.

[beorn7]

@zenador here is the promised fix.

[beorn7]

This should fix #13671.

[juliusv]

I had to read that multiple times before I think I understood it. So here's an illustration of the issue, I hope it's correct:

[juliusv]

👍

[beorn7]

I had scribbled a graph on paper, but it was not legible enough to put a scan here. 😁

@juliusv your graph is almost correct. But the zero point is actually far enough from the first sample (more than "sampleInterval * 1.1") that it would fall back to "sampleInterval / 2". So the problem only happens if the zero point is closer than "sampleInterval * 1.1" but farther than "sampleInterval / 2".

[juliusv]

Ah, thanks for clarifying! Indeed so complicated to think about!