✨ WIP Add feature gate marker support for conditional CRD field inclusion

[wazery]

Implements +kubebuilder:feature-gate=<gate-name> marker that allows fields to be conditionally included in generated CRDs based on enabled feature gates.

• Add centralized featuregate package with evaluation and parsing logic
• Implement CRD feature gate markers for conditional field inclusion
• Add RBAC feature gate support with multiple gate evaluation
• Extend Webhook generator with feature gate capabilities
• Include comprehensive test coverage and golden output files

Addresses #1238

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[JoelSpeed]

I'm keen to seen progress on this issue, thanks @wazery for starting the push forward.

Feature gating is complex when it comes to CRDs. I'd be keen to see if we can also work out a plan for how we will gate specific markers, is that something we can align on as a project before we move forward?

Specifically I've seen needs for feature gated different enum sets, different feature gated XValidations and different values for things like Min/Max items that are feature gated.

But perhaps we actually need to be able to support gating all of the various markers we support?

/ok-to-test

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@wazery: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-controller-tools-test-main	4d70735	link	true	/test pull-controller-tools-test-main

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[wazery]

I'm keen to seen progress on this issue, thanks @wazery for starting the push forward.

Feature gating is complex when it comes to CRDs. I'd be keen to see if we can also work out a plan for how we will gate specific markers, is that something we can align on as a project before we move forward?

Specifically I've seen needs for feature gated different enum sets, different feature gated XValidations and different values for things like Min/Max items that are feature gated.

But perhaps we actually need to be able to support gating all of the various markers we support?

/ok-to-test

Thanks a lot Joel for your review, I truly appreciate it and learn much from it.

As you can see in the current implementation it's very initial, and allows only binary field inclusion so either the entire field is in or out. Yet we can enhance the implementation further to address your cases:

• Feature-gated enum values
• Feature-gated validation rules (XValidations)
• Feature-gated marker values (Min/MaxItems, etc.)
• Conditional marker application

For example:

Option A: Inline Feature Gate Syntax

// Feature-gated enum values
// +kubebuilder:validation:Enum=stable;production
// +kubebuilder:validation:Enum=alpha;beta;experimental,featureGate=alpha

// Feature-gated validation rules  
// +kubebuilder:validation:XValidation=rule="self.size > 0"
// +kubebuilder:validation:XValidation=rule="self.advanced == true",featureGate=alpha

// Feature-gated min/max values
// +kubebuilder:validation:MinItems=1
// +kubebuilder:validation:MinItems=5,featureGate=strict

Option B: Dedicated Feature Gate Markers

// +kubebuilder:validation:Enum=stable;production
// +kubebuilder:validation:Enum:featureGate=alpha,value=alpha;beta;experimental

// +kubebuilder:validation:XValidation=rule="self.size > 0"  
// +kubebuilder:validation:XValidation:featureGate=alpha,rule="self.advanced == true"

Option C: Block-based Feature Gating

// +kubebuilder:feature-gate:alpha
// +kubebuilder:validation:Enum=alpha;beta;experimental  
// +kubebuilder:validation:XValidation=rule="self.advanced == true"
// +kubebuilder:feature-gate:end

// +kubebuilder:validation:Enum=stable;production  // always applied

I love to hear your thoughts on how do you think we have to move forward on this PR, and truly appreciated your help 🙇.

[JoelSpeed]

From your suggestions, I'd be interested to see if we can move towards Option A, though I don't know how simple that is for markers where we currently don't have keyed values.

So for example, in XValidation, it's easy to add another value as it's already a multi-value marker. For the single value markers, adding an optional additional field isn't something I've looked into (not recently at least), but from the UX perspective sounds like the best IMO

Interested to see if other maintainers have opinions on this project @alvaroaleman @sbueringer (and @everettraven who is a colleague who helps me with API review at RH)