Add feature gate discovery tests and improve parser

Author: wazery

FEATURE_GATES.md

@@ -95,6 +95,7 @@
     - [Object/DeepCopy](./reference/markers/object.md)
     - [RBAC](./reference/markers/rbac.md)
     - [Scaffold](./reference/markers/scaffold.md)
+    - [Feature Gates](./reference/markers/feature-gates.md)
 
   - [controller-gen CLI](./reference/controller-gen.md)
   - [completion](./reference/completion.md)

docs/book/src/SUMMARY.md

@@ -38,8 +38,8 @@ controller-gen:
 
 - `make manifests` generates Kubernetes object YAML, like
   [CustomResourceDefinitions](./markers/crd.md),
-  [WebhookConfigurations](./markers/webhook.md), and [RBAC
-  roles](./markers/rbac.md).
+  [WebhookConfigurations](./markers/webhook.md), [RBAC
+  roles](./markers/rbac.md), and [Feature Gates](./markers/feature-gates.md).
 
 - `make generate` generates code, like [runtime.Object/DeepCopy
   implementations](./markers/object.md).

docs/book/src/reference/markers.md

@@ -0,0 +1,136 @@
+# Feature Gates
+
+Feature gates allow you to enable or disable experimental features in your Kubebuilder controllers. This is similar to how Kubernetes core uses feature gates to manage experimental functionality.
+
+## Overview
+
+Feature gates provide a mechanism to:
+- Control the availability of experimental features
+- Enable gradual rollout of new functionality
+- Maintain backward compatibility during API evolution
+
+## Usage
+
+### Marking Fields with Feature Gates
+
+Use the `+feature-gate` marker to mark experimental fields in your API types:
+
+```go
+type MyResourceSpec struct {
+    // Standard field
+    Name string `json:"name"`
+
+    // Experimental field that requires the "experimental-bar" feature gate
+    // +feature-gate experimental-bar
+    Bar *string `json:"bar,omitempty"`
+}
+```
+
+### Running with Feature Gates
+
+Enable feature gates when running your controller:
+
+```bash
+# Enable a single feature gate
+./manager --feature-gates=experimental-bar
+
+# Enable multiple feature gates
+./manager --feature-gates=experimental-bar,advanced-features
+
+# Mixed enabled/disabled states
+./manager --feature-gates=experimental-bar=true,advanced-features=false
+```
+
+### Feature Gate Formats
+
+The `--feature-gates` flag accepts:
+- `feature1` - Enables feature1 (defaults to true)
+- `feature1=true` - Explicitly enables feature1
+- `feature1=false` - Explicitly disables feature1
+- `feature1,feature2` - Enables both features
+- `feature1=true,feature2=false` - Mixed states
+
+## Best Practices
+
+### Naming Conventions
+
+Use descriptive, lowercase names with hyphens:
+- ✅ `experimental-bar`
+- ✅ `advanced-features`
+- ❌ `ExperimentalBar`
+- ❌ `advanced_features`
+
+### Documentation
+
+Always document feature-gated fields:
+
+```go
+// Bar is an experimental field that requires the "experimental-bar" feature gate
+// +feature-gate experimental-bar
+// +optional
+Bar *string `json:"bar,omitempty"`
+```
+
+### Gradual Rollout Strategy
+
+1. **Alpha**: Feature behind feature gate
+2. **Beta**: Feature enabled by default, gate for opt-out
+3. **Stable**: Remove feature gate, feature always available
+
+## Future Integration
+
+When [controller-tools supports feature gates](https://github.com/kubernetes-sigs/controller-tools/issues/1238), you'll be able to use:
+
+```go
+// +kubebuilder:feature-gate=experimental-bar
+// +optional
+Bar *string `json:"bar,omitempty"`
+```
+
+This will automatically exclude the field from CRD schemas when the feature gate is disabled.
+
+## Examples
+
+### Basic Example
+
+```go
+type CronJobSpec struct {
+    // Standard fields
+    Schedule string `json:"schedule"`
+    
+    // Experimental timezone support
+    // +feature-gate timezone-support
+    Timezone *string `json:"timezone,omitempty"`
+}
+```
+
+### Controller Logic
+
+```go
+func (r *CronJobReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
+    // Check if timezone feature is enabled
+    if featureGates.IsEnabled("timezone-support") {
+        // Use timezone-aware scheduling
+        return r.reconcileWithTimezone(ctx, req)
+    }
+    
+    // Fall back to standard scheduling
+    return r.reconcileStandard(ctx, req)
+}
+```
+
+## Troubleshooting
+
+### Common Issues
+
+1. **Feature gate not discovered**: Ensure the `+feature-gate` marker is on the line before the field
+2. **Invalid feature gate name**: Use lowercase with hyphens only
+3. **Validation errors**: Check that all specified gates are available
+
+### Debugging
+
+Enable debug logging to see feature gate discovery:
+
+```bash
+./manager --feature-gates=experimental-bar --zap-log-level=debug
+```