kubernetes: creationTimestamp: null is invalid since it isn't a string

What happened?

There seems to be a discrepancy between the Kubernetes openapi spec and the Go package with the creationTimestamp

What did you expect to happen?

CRDs generated by controller-gen which uses k8s.io/apimachinery/pkg/apis/meta/v1 to be valid according to the Kubernetes openapi spec.

How can we reproduce it (as minimally and precisely as possible)?

# wget https://github.com/fluxcd/source-controller/releases/download/v0.22.5/source-controller.crds.yaml
# kubeconform -schema-location default -schema-location "https://raw.githubusercontent.com/yannh/kubernetes-json-schema/master/master/customresourcedefinition.json" source-controller.crds.yaml
source-controller.crds.yaml - CustomResourceDefinition gitrepositories.source.toolkit.fluxcd.io is invalid: For field metadata.creationTimestamp: Invalid type. Expected: string, given: null
source-controller.crds.yaml - CustomResourceDefinition buckets.source.toolkit.fluxcd.io is invalid: For field metadata.creationTimestamp: Invalid type. Expected: string, given: null
source-controller.crds.yaml - CustomResourceDefinition helmrepositories.source.toolkit.fluxcd.io is invalid: For field metadata.creationTimestamp: Invalid type. Expected: string, given: null
source-controller.crds.yaml - CustomResourceDefinition helmcharts.source.toolkit.fluxcd.io is invalid: For field metadata.creationTimestamp: Invalid type. Expected: string, given: null

Anything else we need to know?

At first I raised an issue at https://github.com/fluxcd/flux2/issues/2623 since they seemed to be the culprit. But received a response that they use controller-gen to generate the CRDs. So I looked into controller-gen and they also do not set the creationTimestamp they import k8s.io/apimachinery/pkg/apis/meta/v1 for the generation of the metadata (where the creationTimestamp is part of)(source).

So now the issue, in the package documentation (link) of k8s.io/apimachinery/pkg/apis/meta/v1 it says the following about the creationTimestamp

// CreationTimestamp is a timestamp representing the server time when this object was
// created. It is not guaranteed to be set in happens-before order across separate operations.
// Clients may not set this value. It is represented in RFC3339 form and is in UTC.
//
// Populated by the system.
// Read-only.
// Null for lists.
// More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#metadata
// +optional
Creation[Time](https://pkg.go.dev/k8s.io/apimachinery/pkg/apis/meta/v1#Time)stamp Time `json:"creationTimestamp,omitempty" protobuf:"bytes,8,opt,name=creationTimestamp"`

So there is something in the docblock about Null for lists(?) but in the openapi spec of Kubernetes is this Time resource marked as string:

"io.k8s.apimachinery.pkg.apis.meta.v1.Time": {
      "description": "Time is a wrapper around time.Time which supports correct marshaling to YAML and JSON.  Wrappers are provided for many of the factory methods that the time package offers.",
      "format": "date-time",
      "type": "string"
},

https://raw.githubusercontent.com/kubernetes/kubernetes/master/api/openapi-spec/swagger.json

So now, what is wrong here? Is it indeed allowed to be a string or null and should the openapi spec be an enum? Or is there something ink8s.io/apimachinery/pkg/apis/meta/v1 wrong? Or should controller-gen explictly set the creationTimestamp to an string? The last one seems kinda wrong to me since it seems to me that the k8s.io/apimachinery/pkg/apis/meta/v1 should return data valid to the api spec.

I am sorry for all the text and if I did not understand something correctly 😃 I don’t develop in golang (or at all really) and the relation between the package and the openapi spec is quite vague for me.

I opened the issue here since it seems an issue affecting multiple things within the kubernetes namespace (golang package, openapi spec in this repo).

Kubernetes version

kubectl version Client Version: version.Info{Major:"1", Minor:"23", GitVersion:"v1.23.5", GitCommit:"c285e781331a3785a7f436042c65c5641ce8a9e9", GitTreeState:"clean", BuildDate:"2022-03-16T15:51:05Z", GoVersion:"go1.17.8", Compiler:"gc", Platform:"darwin/arm64"} Server Version: version.Info{Major:"1", Minor:"22", GitVersion:"v1.22.6", GitCommit:"5963a7a5570a1b8c977c31ced503cb20f0ef6e8c", GitTreeState:"clean", BuildDate:"2022-03-10T07:59:42Z", GoVersion:"go1.16.12", Compiler:"gc", Platform:"linux/amd64"}