crd: allow specifying spec.preserveUnknownFields

Reinstate crd:preserveUnknownFields option removed by kubernetes-sigs#607
to allow specifying the value of deprecated spec.preserveUnknownFields
CRD field.

This is useful for updating CRDs that were automatically converted from
v1beta1 version which had true as a default value and resulted in:
```
$ kubectl get crd foo.bar -o yaml | grep preserveUnknownFields
  preserveUnknownFields: true
    message: 'spec.preserveUnknownFields: Invalid value: true: must be false'
```

Fixes kubernetes-sigs#476

pkg/crd/gen.go

@@ -85,6 +85,13 @@ type Generator struct {
 
 	// Year specifies the year to substitute for " YEAR" in the header file.
 	Year string `marker:",optional"`
+
+	// PreserveUnknownFields indicates whether or not we should turn off pruning.
+	//
+	// Specifies spec.preserveUnknownFields value that is false and omitted by default.
+	//
+	// It's required to be false for v1 CRDs.
+	PreserveUnknownFields *bool `marker:",optional"`
 }
 
 func (Generator) CheckFilter() loader.NodeFilter {
@@ -100,6 +107,16 @@ func transformRemoveCRDStatus(obj map[string]interface{}) error {
 	return nil
 }
 
+// transformPreserveUnknownFields adds spec.preserveUnknownFields=value.
+func transformPreserveUnknownFields(value bool) func(map[string]interface{}) error {
+	return func(obj map[string]interface{}) error {
+		if spec, ok := obj["spec"].(map[interface{}]interface{}); ok {
+			spec["preserveUnknownFields"] = value
+		}
+		return nil
+	}
+}
+
 func (g Generator) Generate(ctx *genall.GenerationContext) error {
 	parser := &Parser{
 		Collector: ctx.Collector,
@@ -146,6 +163,14 @@ func (g Generator) Generate(ctx *genall.GenerationContext) error {
 	}
 	headerText = strings.ReplaceAll(headerText, " YEAR", " "+g.Year)
 
+	yamlOpts := []*genall.WriteYAMLOptions{
+		genall.WithTransform(transformRemoveCRDStatus),
+		genall.WithTransform(genall.TransformRemoveCreationTimestamp),
+	}
+	if g.PreserveUnknownFields != nil {
+		yamlOpts = append(yamlOpts, genall.WithTransform(transformPreserveUnknownFields(*g.PreserveUnknownFields)))
+	}
+
 	for _, groupKind := range kubeKinds {
 		parser.NeedCRDFor(groupKind, g.MaxDescLen)
 		crdRaw := parser.CustomResourceDefinitions[groupKind]
@@ -171,7 +196,7 @@ func (g Generator) Generate(ctx *genall.GenerationContext) error {
 			} else {
 				fileName = fmt.Sprintf("%s_%s.%s.yaml", crdRaw.Spec.Group, crdRaw.Spec.Names.Plural, crdVersions[i])
 			}
-			if err := ctx.WriteYAML(fileName, headerText, []interface{}{crd}, genall.WithTransform(transformRemoveCRDStatus), genall.WithTransform(genall.TransformRemoveCreationTimestamp)); err != nil {
+			if err := ctx.WriteYAML(fileName, headerText, []interface{}{crd}, yamlOpts...); err != nil {
 				return err
 			}
 		}

pkg/crd/gen_integration_test.go

@@ -119,6 +119,32 @@ var _ = Describe("CRD Generation proper defaulting", func() {
 		expectedOut := string(expectedFileFoos) + string(expectedFileZoos)
 		Expect(out.buf.String()).To(Equal(expectedOut), cmp.Diff(out.buf.String(), expectedOut))
 	})
+
+	It("should add preserveUnknownFields=false when specified", func() {
+		By("calling Generate")
+		no := false
+		gen := &crd.Generator{
+			CRDVersions:           []string{"v1"},
+			PreserveUnknownFields: &no,
+		}
+		Expect(gen.Generate(ctx)).NotTo(HaveOccurred())
+
+		By("searching preserveUnknownFields")
+		Expect(out.buf.String()).To(ContainSubstring("preserveUnknownFields: false"))
+	})
+
+	It("should add preserveUnknownFields=true when specified", func() {
+		By("calling Generate")
+		yes := true
+		gen := &crd.Generator{
+			CRDVersions:           []string{"v1"},
+			PreserveUnknownFields: &yes,
+		}
+		Expect(gen.Generate(ctx)).NotTo(HaveOccurred())
+
+		By("searching preserveUnknownFields")
+		Expect(out.buf.String()).To(ContainSubstring("preserveUnknownFields: true"))
+	})
 })
 
 // fixAnnotations fixes the attribution annotation for tests.