Fix error for strategic merge patch of custom resources

[nikhita]

Fixes #50037.

We need the go struct tags patchMergeKey and patchStrategy for fields that support a strategic merge patch. For native resources, we can easily figure out these tags since we know the fields.

Because custom resources are decoded as Unstructured and because we're missing the metadata about how to handle each field in a strategic merge patch, we can't find the go struct tags. Hence, we can't easily do a strategic merge for custom resources.

So we should fail fast and return an error.

Release note:

NONE

/cc @sttts @deads2k @ncdc

[k8s-ci-robot]

Hi @nikhita. Thanks for your PR.

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[nikhita]

Now, the error message is shown as:

$ cat crd.yaml
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  version: v1
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct

$ kubectl create -f crd.yaml
customresourcedefinition "crontabs.stable.example.com" created

$ cat crontab-object.yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image

$ kubectl create -f crontab-object.yaml
crontab "my-new-cron-object" created

$ kubectl patch crontab my-new-cron-object -p '{"spec":{"image":"new-image"}}'
Error from server: cannot apply strategic merge patch for custom resources, try --type merge

[dims]

/ok-to-test

[nikhita]

/retest

[sttts]

Isn't the strategy patch logic without any metadata just the classical non-strategic patch? What does the user see now?

In general, I would prefer not to mention custom resources here in apimachinery. An error constant in vendor/k8s.io/apimachinery/pkg/util/mergepatch/errors.go with "ErrUnsupportedPatchFormat" would make more sense.

[nikhita]

Done.

[nikhita]

What does the user see now?

The user sees: Error from server: unable to find api field in struct Unstructured for the json field "spec".

After this update, it is Error from server: strategic merge patch format is not supported.

[ncdc]

@nikhita feel like adding a unit test?

[nikhita]

@ncdc Will do!

Also, maybe we should hold this off until #53379 (comment) is resolved (working on it).

[apelisse]

Looks good to me.

[nikhita]

Added a test and updated the comment to add more detail into why we can't support strategic merge patch for custom resources.

[sttts]

would _, ok := dataStruct.(Unstructured); ok be the same?

[nikhita]

_, ok := dataStruct.(*unstructured.Unstructured); ok would be the same (with the pointer). 👍

updated.

[nikhita]

/retest

[sttts]

/lgtm
/approve

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nikhita, sttts

Associated issue: 50037

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Needs approval from an approver in each of these OWNERS Files:

• staging/src/k8s.io/apimachinery/pkg/util/mergepatch/OWNERS [sttts]
• staging/src/k8s.io/apimachinery/pkg/util/strategicpatch/OWNERS [sttts]
• staging/src/k8s.io/apiserver/OWNERS [sttts]

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[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 54800, 53898, 54812, 54921, 53558). If you want to cherry-pick this change to another branch, please follow the instructions here.