[usage] Fix usage report overflow

[easyCZ]

Description

My testing dataset was not large enough, and we'd run into int64 overflows for very long running instances. Using Seconds as the baseline unit fixes that.

Related Issue(s)

Fixes #

How to test

Unit tests

Release Notes

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Documentation

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[jankeromnes]

Why the overkill floating-point precision on seconds? 🤔 Rounded-up seconds would probably be precise enough, right?

[easyCZ]

Wanted to not introduce any rounding before we discuss it and capture so that we can then explain to customers.

[jankeromnes]

Credits are likely to map to hours anyway. I think rounding to the next second is extremely fine. 👍

Plus, it's a lot more readable for humans, and doesn't have any built-in micro-imprecision (remember that 0.1 + 0.2 === 0.30000000000000004 🙈)

[easyCZ]

Let's do that in a follow-up PR.

[jankeromnes]

Mmf, approving to unblock progress, but I really don't think we want float64 precision on our seconds.

Please switch to rounding runtimes to the next second in a follow-up PR. 🙏

[jankeromnes]

This is completely unreadable 🙄

[jankeromnes]

Drive-by comment: So the maxStopTime logic makes sense (if we ask for the usage on 2nd day of month, and some instance is not stopped yet, we can count like if it was stopped right now to get the current usage up to now).

However, you also need to trim:

• i.CreationTime to MAX(beginningOfMonth, i.CreationTime)
• any existing i.StoppedTime to MIN(maxStopTime, i.StoppedTime)

The reason is that, if the instance has been running through May and into June, and we're currently computing the usage of June -- we don't want to double-count any runtime from May (which was presumably already counted and invoiced).

Same for running a usage report retroactively -- if an instance was started in April, and ran well into May, and you're getting the usage report of April -- you only want the usage that happened in April.

[easyCZ]

The trimming already happens here, after the workspace instances are loaded and validated.

Here, it is a general purpose method which works on the data on the struct, the trimming is specific to the usage report for billing and therefore done there.

[jankeromnes]

Aha, cool! 🎯

gitpod/components/usage/pkg/controller/reconciler.go

Lines 288 to 294 in e3f9ca5

	if instance.CreationTime.Time().Before(maximumStart) {
	instance.CreationTime = db.NewVarcharTime(maximumStart)
	}
	if instance.StoppedTime.Time().After(minimumStop) {
	instance.StoppedTime = db.NewVarcharTime(minimumStop)
	}

I guess these should actually be called minimumStart and maximumStop 😅 but the logic looks correct. 👍

[easyCZ]

Follow-up in #10591