Add XValidations CRD marker for KEP-2876 support.

This allows generation of the x-kubernetes-validations extension for
expression-based validation rules.

pkg/crd/markers/validation.go

@@ -67,6 +67,7 @@ var ValidationMarkers = mustMakeAllWithPrefix("kubebuilder:validation", markers.
 	XPreserveUnknownFields{},
 	XEmbeddedResource{},
 	XIntOrString{},
+	XValidations{},
 )
 
 // FieldOnlyMarkers list field-specific validation markers (i.e. those markers that don't make
@@ -251,6 +252,17 @@ type XIntOrString struct{}
 // to be used only as a last resort.
 type Schemaless struct{}
 
+// +controllertools:marker:generateHelp:category="CRD validation"
+// XValidations marks a field as requiring a value for which a given
+// expression evaluates to true.
+//
+// This marker may be repeated to specify multiple expressions, all of
+// which must evaluate to true.
+type XValidations struct {
+	Rule    string
+	Message string `marker:",optional"`
+}
+
 func (m Maximum) ApplyToSchema(schema *apiext.JSONSchemaProps) error {
 	if schema.Type != "integer" {
 		return fmt.Errorf("must apply maximum to an integer")
@@ -428,3 +440,11 @@ func (m XIntOrString) ApplyToSchema(schema *apiext.JSONSchemaProps) error {
 }
 
 func (m XIntOrString) ApplyFirst() {}
+
+func (m XValidations) ApplyToSchema(schema *apiext.JSONSchemaProps) error {
+	schema.XValidations = append(schema.XValidations, apiext.ValidationRule{
+		Rule:    m.Rule,
+		Message: m.Message,
+	})
+	return nil
+}

pkg/crd/testdata/cronjob_types.go

@@ -181,6 +181,11 @@ type CronJobSpec struct {
 
 	// Maps of arrays of things-that-aren’t-strings are permitted
 	MapOfArraysOfFloats map[string][]bool `json:"mapOfArraysOfFloats,omitempty"`
+
+	// Test of the expression-based validation rule marker, with optional message.
+	// +kubebuilder:validation:XValidations:rule="self.size() % 2 == 0",message="must have even length"
+	// +kubebuilder:validation:XValidations:rule="true"
+	StringWithEvenLength string `json:"stringWithEvenLength,omitempty"`
 }
 
 type ContainsNestedMap struct {

pkg/crd/testdata/testdata.kubebuilder.io_cronjobs.yaml

@@ -7259,6 +7259,14 @@ spec:
                   type: array
                 description: This tests string slices are allowed as map values.
                 type: object
+              stringWithEvenLength:
+                description: Test of the expression-based validation rule marker,
+                  with optional message.
+                type: string
+                x-kubernetes-validations:
+                - message: must have even length
+                  rule: self.size() % 2 == 0
+                - rule: "true"
               structWithSeveralFields:
                 description: A struct that can only be entirely replaced
                 properties: