Fix kubectl drain error handling bug.

[brianpursley]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fixed a bug where kubectl drain would consider a pod as having been deleted if an error occurs while calling the API.

This can happen if you reboot the master node(s) while draining, for example. In this case an error occurs, but the pod has not actually been deleted yet.

The problem occurs because the client-go returns an empty struct instead of nil, even when there is an error, as shown here:

kubernetes/staging/src/k8s.io/client-go/kubernetes/typed/core/v1/pod.go

Lines 75 to 85 in 6aac45f

	func (c *pods) Get(ctx context.Context, name string, options metav1.GetOptions) (result *v1.Pod, err error) {
	result = &v1.Pod{}
	err = c.client.Get().
	Namespace(c.ns).
	Resource("pods").
	Name(name).
	VersionedParams(&options, scheme.ParameterCodec).
	Do(ctx).
	Into(result)
	return
	}

The kubectl drain code assumed that an error did not occur if the returned pod was not nil, which is incorrect.

This PR fixes that bug and updates the existing unit tests to use an empty struct instead of nil, matching what is actually happening. I also added a new test case to cover the case where a pod is returned, but the UID is different.

Which issue(s) this PR fixes:

Fixes kubernetes/kubectl#1532

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fixed a bug where kubectl drain would consider a pod as having been deleted if an error occurs while calling the API.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[brianpursley]

/hold for review

/cc @eddiezane
/cc @ardaguclu

[ardaguclu]

/triage accepted
/priority backlog

[ardaguclu]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ardaguclu, brianpursley

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• staging/src/k8s.io/kubectl/OWNERS [ardaguclu,brianpursley]

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[brianpursley]

/unhold

[brianpursley]

@ardaguclu can you lgtm if you think this is good to go?

[ardaguclu]

Thanks for the fix
/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 167b8a70e9b51f83d5cc6296edc0f413bd47398b