fix: descheduler_loop_duration_seconds has wrong value

[Abirdcfly]

see golang/go#60048

I learned this change from https://github.com/golang/tools/blob/9abb02c1b5524bf655a0985bc101496a6b53ff71/internal/gocommand/invoke.go#L265, thanks to author https://github.com/heschi 🙏

[a7i]

Nice catch and thanks for the references/links! 🏆

/ok-to-test
/lgtm

[Abirdcfly]

/test pull-descheduler-verify-master

[Abirdcfly]

please assign ingvagabund for approval

/assign @ingvagabund

thanks.

[ingvagabund]

Should not this be more like:

loopStartDuration := time.Now()
defer func(loopStartDuration time.Time) {
	metrics.DeschedulerLoopDuration.With(map[string]string{}).Observe(time.Since(loopStartDuration).Seconds())
}(loopStartDuration)

?
Or just:

loopStartDuration := time.Now()
defer func() {
	metrics.DeschedulerLoopDuration.With(map[string]string{}).Observe(time.Since(loopStartDuration).Seconds())
}()

[Abirdcfly]

I think both of these changes work.

It's straightforward to use defer func(...) {...} (time.Now()) because that's the only place the loopStartDuration variable is used, and it doesn't seem like it needs an extra variable defined.

[ingvagabund]

time.Now() will have different value for loopStartDuration := time.Now() and when defer is invoked. Unless I incorrectly understand how the defer statement invokes the function/code. My assumption is anything after defer gets invoke right before a function/method exits.

[ingvagabund]

https://go.dev/tour/flowcontrol/12 says:

The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.

Interesting.

[ingvagabund]

/approve

[k8s-ci-robot]

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