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apiextensions: add subresources for custom resources
Fixes#38113Fixes#58778
**Related**:
- Proposal: https://github.com/kubernetes/community/pull/913
- For custom resources to work with `kubectl scale`: https://github.com/kubernetes/kubernetes/pull/58283
**Add types**:
- Add `CustomResourceSubResources` type to CRD.
- Fix proto generation for `CustomResourceSubResourceStatus`: https://github.com/kubernetes/kubernetes/pull/55970.
- Add feature gate for `CustomResourceSubResources`.
- Update CRD strategy: if feature gate is disabled, this feature is dropped (i.e. set to `nil`).
- Add validation for `CustomResourceSubResources`:
- `SpecReplicasPath` should not be empty and should be a valid json path under `.spec`. If there is no value under the given path in the CustomResource, the `/scale` subresource will return an error on GET.
- `StatusReplicasPath` should not be empty and should be a valid json path under `.status`. If there is no value under the given path in the CustomResource, the status replica value in the /scale subresource will default to 0.
- If present, `LabelSelectorPath` should be a valid json path. If there is no value under `LabelSelectorPath` in the CustomResource, the status label selector value in the `/scale` subresource will default to the empty string.
- `ScaleGroupVersion` should be `autoscaling/v1`.
- If `CustomResourceSubResources` is enabled, only `properties` is allowed under the root schema for CRD validation.
**Add status and scale subresources**:
- Use helper functions from `apimachinery/pkg/apis/meta/v1/unstructured/helpers.go`.
- Improve error handling: https://github.com/kubernetes/kubernetes/pull/56563, https://github.com/kubernetes/kubernetes/pull/58215.
- Introduce Registry interface for storage.
- Update storage:
- Introduce `CustomResourceStorage` which acts as storage for the custom resource and its status and scale subresources. Note: storage for status and scale is only enabled when the feature gate is enabled _and_ the respective fields are enabled in the CRD.
- Introduce `StatusREST` and its `New()`, `Get()` and `Update()` methods.
- Introduce `ScaleREST` and its `New()`, `Get()` and `Update()` methods.
- Get and Update use the json paths from the CRD and use it to return an `autoscaling/v1.Scale` object.
- Update strategy:
- In `PrepareForCreate`,
- Clear `.status`.
- Set `.metadata.generation` = 1
- In `PrepareForUpdate`,
- Do not update `.status`.
- If both the old and new objects have `.status` and it is changed, set it back to its old value.
- If the old object has a `.status` but the new object doesn't, set it to the old value.
- If old object did not have a `.status` but the new object does, delete it.
- Increment generation if spec changes i.e. in the following cases:
- If both the old and new objects had `.spec` and it changed.
- If the old object did not have `.spec` but the new object does.
- If the old object had a `.spec` but the new object doesn't.
- In `Validate` and `ValidateUpdate`,
- ensure that values at `specReplicasPath` and `statusReplicasPath` are >=0 and < maxInt32.
- make sure there are no errors in getting the value at all the paths.
- Introduce `statusStrategy` with its methods.
- In `PrepareForUpdate`:
- Do not update `.spec`.
- If both the old and new objects have `.spec` and it is changed, set it back to its old value.
- If the old object has a `.spec` but the new object doesn't, set it to the old value.
- If old object did not have a `.spec` but the new object does, delete it.
- Do not update `.metadata`.
- In `ValidateStatusUpdate`:
- For CRD validation, validate only under `.status`.
- Validate value at `statusReplicasPath` as above. If `labelSelectorPath` is a path under `.status`, then validate it as well.
- Plug into the custom resource handler:
- Store all three storage - customResource, status and scale in `crdInfo`.
- Use the storage as per the subresource in the request.
- Use the validator as per the subresource (for status, only use the schema for `status`, if present).
- Serve the endpoint as per the subresource - see `serveResource`, `serveStatus` and `serveScale`.
- Update discovery by adding the `/status` and `/scale` resources, if enabled.
**Add tests**:
- Add unit tests in `etcd_test.go`.
- Add integration tests.
- In `subresources_test.go`, use the [polymporphic scale client](https://github.com/kubernetes/kubernetes/tree/master/staging/src/k8s.io/client-go/scale) to get and update `Scale`.
- Add a test to check everything works fine with yaml in `yaml_test.go`.
**Release note**:
```release-note
`/status` and `/scale` subresources are added for custom resources.
```
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