minikube update-context: Fix nil pointer dereference

[superbrothers]

Fixes #8988

[k8s-ci-robot]

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[medyagh]

I am glad you are doing a PR to improve this, could you please add a subtest to our exiting UpdateContext integration test?

https://github.com/medyagh/minikube/blob/5c73732d767154d168604dac8a45d5e534978a33/test/integration/functional_test.go#L1010

you could do a subtest, by passing a KUBECONFIG env to the command that points to a problamtic kubeconfig.

for example, you can add something like this
https://github.com/medyagh/minikube/blob/5c73732d767154d168604dac8a45d5e534978a33/test/integration/functional_test.go#L1010-L1014
cmd= exec.CommandContext(ctx, Target(), "-p", profile, "update-context", "--alsologtostderr", "-v=2")
cmd.Env =..... (add a KUBECONFIG env here...you can create a dummy temp file or add the probalmatic kubeconfig to the testdata folder.
rr, err := Run(t,cmd)
if err != nil {
t.Errorf("failed to run minikube update-context: args %q: %v", rr.Command(), err)
}

[superbrothers]

@medyagh OK, I'll add a subtest for Update Context, but we need to decide update-context command's behavior first.

If there is no minikube cluster in the kubeconfig (including if no clusters), add cluster, user and context settings to the kubeconfig, and set minikube to the current-context. This is the current behavior, but according to TestUpdateIP in pkg/minikube/kubeconfig_test.go, update-context command is originally expected to result in an error.

func TestUpdateIP(t *testing.T) {

	var tests = []struct {
		description string
		hostname    string
		port        int
		existing    []byte
		err         bool
		status      bool
		expCfg      []byte
	}{
		{
			description: "no minikube cluster",
			hostname:    "192.168.10.100",
			port:        8080,
			existing:    kubeConfigWithoutHTTPS,
			err:         true,
			expCfg:      kubeConfigWithoutHTTPS,
		},

[superbrothers]

@medyagh I updated a subtest to our exiting UpdateContext integration test at eb01a2e.

$ env TEST_ARGS="-test.run=TestFunctional/parallel/UpdateContextCmd --profile=minikube --cleanup=false" make integration
go test -ldflags="-X k8s.io/minikube/pkg/version.version=v1.12.3 -X k8s.io/minikube/pkg/version.isoVersion=v1.12.2 -X k8s.io/minikube/pkg/version.isoPath=minikube/iso -X k8s.io/minikube/pkg/version.gitCommitID="3ecf887959389a85a04f4b3ba34f83f12b88536f-dirty" -X k8s.io/minikube/pkg/version.storageProvisionerVersion=v2" -v -test.timeout=90m ./test/integration --tags="integration container_image_ostree_stub containers_image_openpgp go_getter_nos3 go_getter_nogcs" -test.run=TestFunctional/parallel/UpdateContextCmd --profile=minikube --cleanup=false
Found 16 cores, limiting parallelism with --test.parallel=9
=== RUN   TestFunctional
=== PAUSE TestFunctional
=== CONT  TestFunctional
=== RUN   TestFunctional/parallel
=== RUN   TestFunctional/parallel/UpdateContextCmd
=== PAUSE TestFunctional/parallel/UpdateContextCmd
=== CONT  TestFunctional/parallel/UpdateContextCmd
=== RUN   TestFunctional/parallel/UpdateContextCmd/no_changes
=== PAUSE TestFunctional/parallel/UpdateContextCmd/no_changes
=== RUN   TestFunctional/parallel/UpdateContextCmd/no_minikube_cluster
=== PAUSE TestFunctional/parallel/UpdateContextCmd/no_minikube_cluster
=== RUN   TestFunctional/parallel/UpdateContextCmd/no_clusters
=== PAUSE TestFunctional/parallel/UpdateContextCmd/no_clusters
=== CONT  TestFunctional/parallel/UpdateContextCmd/no_changes
=== CONT  TestFunctional/parallel/UpdateContextCmd/no_clusters
=== CONT  TestFunctional/parallel/UpdateContextCmd/no_minikube_cluster
=== CONT  TestFunctional/parallel/UpdateContextCmd/no_changes
    functional_test.go:1082: (dbg) Run:  out/minikube -p minikube update-context --alsologtostderr -v=2
=== CONT  TestFunctional/parallel/UpdateContextCmd/no_minikube_cluster
    functional_test.go:1082: (dbg) Run:  out/minikube -p minikube update-context --alsologtostderr -v=2
=== CONT  TestFunctional/parallel/UpdateContextCmd/no_clusters
    functional_test.go:1082: (dbg) Run:  out/minikube -p minikube update-context --alsologtostderr -v=2
--- PASS: TestFunctional (0.39s)
    --- PASS: TestFunctional/parallel (0.00s)
        --- PASS: TestFunctional/parallel/UpdateContextCmd (0.00s)
            --- PASS: TestFunctional/parallel/UpdateContextCmd/no_changes (0.39s)
            --- PASS: TestFunctional/parallel/UpdateContextCmd/no_minikube_cluster (0.39s)
            --- PASS: TestFunctional/parallel/UpdateContextCmd/no_clusters (0.39s)
PASS
Tests completed in 388.50175ms (result code 0)
ok      k8s.io/minikube/test/integration        1.312s

[superbrothers]

I'll squash the commits after the changes are reviewed.

[tstromberg]

/ok-to-test

[tstromberg]

LGTM pending integration tests

[minikube-pr-bot]

kvm2 Driver
docker Driver

[superbrothers]

Some tests seem to be failing on items not related to this PR.

[tstromberg]

I tested this locally and it works great. Thank you!