Add validation for KubeadmConfigSpec defaulting

[sbueringer]

We repeatedly had the issue that we added new defaulting to KubeadmConfigSpec fields, which lead to rollouts in KCP after CAPI upgrade:

• Upgrade to v1.1 forces rollout of KubeadmControlPlanes #6093
• KCP triggers rollout after upgrade #8124

The goal of this issue is to add some sort of validation that we can run as part of presubmits to ensure this doesn't happen again.

We basically want to validate that all defaulting that we implement for KubeadmConfigSpec is done in DefaultKubeadmConfigSpec and not via OpenAPI.

Some ideas:

• Add a defaulting unit test:
  • It would compare results of local KubeadmConfig defaulting with KubeadmConfig.Default() vs. KubeadmConfig create with envtest
  • => If there is a difference => there was additional defaulting via OpenAPI
• Introduce a new linter. Options:
  • check for default field in final CRD YAMLs
  • check for // +kubebuilder:default marker in KubeadmConfigSpec struct and below (controller-tools based linter)

/kind feature
/area control-plane

[sbueringer]

@vincepri @fabriziopandini @ykakarap

The unit tests sounds sufficient and pretty straightforward to me, WDYT?

[ykakarap]

Yup, unit test looks like a good idea. +1.
Since we will use envtest with the defaulting webhook it will closely reflect the real world case too.

[fabriziopandini]

/triage accepted
/help
💯 this will be very useful!

[k8s-ci-robot]

@fabriziopandini:
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In response to this:

/triage accepted
/help
💯 this will be very useful!

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[sbueringer]

Let's see if some takes this over. I added it to the v1.5 milestone to ensure we'll implement it until then.

I would take over if nobody volunteers until then.

[k8s-triage-robot]

This issue has not been updated in over 1 year, and should be re-triaged.

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/remove-triage accepted

[sbueringer]

/triage accepted