cmd/tailscale/cli: fix configuring partially empty kubeconfig (#11417)

When a user deletes the last cluster/user/context from their
kubeconfig via 'kubectl delete-[cluster|user|context] command,
kubectx sets the relevant field in kubeconfig to 'null'.
This was breaking our conversion logic that was assuming that the field
is either non-existant or is an array.

Updates tailscale/corp#18320

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/tailscale/cli/configure-kube.go

@@ -119,7 +119,7 @@ func updateKubeconfig(cfgYaml []byte, fqdn string) ([]byte, error) {
 
 	var clusters []any
 	if cm, ok := cfg["clusters"]; ok {
-		clusters = cm.([]any)
+		clusters, _ = cm.([]any)
 	}
 	cfg["clusters"] = appendOrSetNamed(clusters, fqdn, map[string]any{
 		"name": fqdn,
@@ -130,7 +130,7 @@ func updateKubeconfig(cfgYaml []byte, fqdn string) ([]byte, error) {
 
 	var users []any
 	if um, ok := cfg["users"]; ok {
-		users = um.([]any)
+		users, _ = um.([]any)
 	}
 	cfg["users"] = appendOrSetNamed(users, "tailscale-auth", map[string]any{
 		// We just need one of these, and can reuse it for all clusters.
@@ -144,7 +144,7 @@ func updateKubeconfig(cfgYaml []byte, fqdn string) ([]byte, error) {
 
 	var contexts []any
 	if cm, ok := cfg["contexts"]; ok {
-		contexts = cm.([]any)
+		contexts, _ = cm.([]any)
 	}
 	cfg["contexts"] = appendOrSetNamed(contexts, fqdn, map[string]any{
 		"name": fqdn,

cmd/tailscale/cli/configure-kube_test.go

@@ -48,6 +48,31 @@ contexts:
 current-context: foo.tail-scale.ts.net
 kind: Config
 users:
+- name: tailscale-auth
+  user:
+    token: unused`,
+		},
+		{
+			name: "all configs, clusters, users have been deleted",
+			in: `apiVersion: v1
+clusters: null
+contexts: null
+kind: Config
+current-context: some-non-existent-cluster
+users: null`,
+			want: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://foo.tail-scale.ts.net
+  name: foo.tail-scale.ts.net
+contexts:
+- context:
+    cluster: foo.tail-scale.ts.net
+    user: tailscale-auth
+  name: foo.tail-scale.ts.net
+current-context: foo.tail-scale.ts.net
+kind: Config
+users:
 - name: tailscale-auth
   user:
     token: unused`,