Fix index out of range error when sorting kubectl get

[mfpierre]

What type of PR is this?
/kind bug

What this PR does / why we need it:

There's an unsafe access to the [0] element of r.objects which can cause this error to happen:

panic: runtime error: index out of range

goroutine 1 [running]:
k8s.io/kubernetes/pkg/kubectl/cmd/get.(*RuntimeSorter).Sort(0xc0008980f0, 0x0, 0x0)
    /private/tmp/kubernetes-cli-20180928-18549-oxqbkv/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/pkg/kubectl/cmd/get/get.go:320 +0x3bc
k8s.io/kubernetes/pkg/kubectl/cmd/get.(*GetOptions).Run(0xc0002ba540, 0x25fa0c0, 0xc000567e60, 0xc000538280, 0xc0000a12c0, 0x1, 0x6, 0x0, 0x0)
    /private/tmp/kubernetes-cli-20180928-18549-oxqbkv/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/pkg/kubectl/cmd/get/get.go:489 +0x1391
k8s.io/kubernetes/pkg/kubectl/cmd/get.NewCmdGet.func1(0xc000538280, 0xc0000a12c0, 0x1, 0x6)
    /private/tmp/kubernetes-cli-20180928-18549-oxqbkv/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/pkg/kubectl/cmd/get/get.go:161 +0x115
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).execute(0xc000538280, 0xc0000a1260, 0x6, 0x6, 0xc000538280, 0xc0000a1260)
    /private/tmp/kubernetes-cli-20180928-18549-oxqbkv/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/github.com/spf13/cobra/command.go:760 +0x2cc
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xc00012b180, 0xc0003a2930, 0x12a05f200, 0xc0007f5f10)
    /private/tmp/kubernetes-cli-20180928-18549-oxqbkv/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/github.com/spf13/cobra/command.go:846 +0x2fd
k8s.io/kubernetes/vendor/github.com/spf13/cobra.(*Command).Execute(0xc00012b180, 0x248e758, 0x3597a80)
    /private/tmp/kubernetes-cli-20180928-18549-oxqbkv/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/github.com/spf13/cobra/command.go:794 +0x2b
main.main()
    /private/tmp/kubernetes-cli-20180928-18549-oxqbkv/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/cmd/kubectl/kubectl.go:50 +0x150

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

fixes a runtime error occuring when sorting the output of `kubectl get` with empty results

[k8s-ci-robot]

Hi @mfpierre. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[neolit123]

thanks for the PR @mfpierre
seems to me like get_test.go would need an update here.

/ok-to-test

[neolit123]

@kubernetes/sig-cli-bugs
/priority important-soon

[mfpierre]

@neolit123 just added a unit test case with an empty list of objects

The test is crashing without the fix

[neolit123]

@mfpierre thanks for the update.

/ok-to-test

[mfpierre]

Looks like trailing some whitespaces caused issues

/retest

[mfpierre]

/test pull-kubernetes-e2e-kops-aws
/test pull-kubernetes-e2e-gce-100-performance

[justinsb]

/lgtm

[mfpierre]

Hey @justinsb thanks for the review, do you think this bug fix can make it into 1.13?

[justinsb]

I think it should, but that's probably a question for sig-cli maintainers. Looks like janetkuo won the github lottery here...

/assign janetkuo

[liggitt]

it's not a regression (bug existed in 1.12), but the fix is contained, the risk is low, and I'd consider it a candidate for cherry-pick.

/milestone v1.13
/priority critical-urgent

[liggitt]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, mfpierre

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