Ensure old revision stays Unreachable after Knative Service update

[SaschaSchwarze0]

Changes

Fixes knative/serving#14115 by changing how IsReady is implemented.

Currently, the IsReady function of the ServerlessService which is returning false if the generation is different in metadata and status.

Due to it temporarily not being ready, the PodAutoscaler is set to SKSReady=Unknown here.

The PodAutoscaler then goes Ready=False because SKSReady is part of Ready here.

The revision inherits the PodAutoscaler status and sets Active=Unknown here.

This causes the PodAutoscaler's Reachability to be set to Unknown here.

Once the PodAutoscaler is not anymore marked as unreachable, the scaling boundary will be set to the min value from the annotation again here. This will cause a revision that is being not needed anymore to scale up again for a short moment which causes additional pods.

I am changing the IsReady of the ServerlessService to only check the condition. This fixes the problem and I have not seen negative impact. Though, it undos the work of IsReady should take ObservedGeneration into account #8004 which sounds to me as if it tried to resolve an abstract issue while I here have a real one. Alternative could be to leave IsReady unchanged und look at the condition directly in kpa.go.

I need somebody with experience in this area to carefully assess if there are side-effects.

/kind bug

Release Note

An Unreachable revision is now not anymore causing additional pods to get created while it is scaled down to 0.

Docs

None

[knative-prow]

Welcome @SaschaSchwarze0! It looks like this is your first PR to knative/networking 🎉

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[knative-prow]

Hi @SaschaSchwarze0. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[dprotaso]

/hold

Let's not change the semantics of IsReady - knative/serving#14115 (comment)