cli: err about extensions for uninstall (#5723)

Fixes #5712

Currently, We don't provide any information w.r.t extensions
when `linkerd uninstall` is ran and extensions are present.

This PR updates the install command to display a error message
asking to uninstall the existing extensions before uninstalling
the core control-plane.

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

cli/cmd/uninstall.go

@@ -34,6 +34,32 @@ This command provides all Kubernetes namespace-scoped and cluster-scoped resourc
 			}
 
 			if !force {
+
+				var fail bool
+				// Retrtieve any installed extensions
+				extensionNamespaces, err := k8sAPI.GetAllNamespacesWithExtensionLabel(cmd.Context())
+				if err != nil {
+					return err
+				}
+
+				// map of the namespace and the extension name
+				// Namespace is used as key so as to support custom namespace installs
+				extensions := make(map[string]string)
+				if len(extensionNamespaces) > 0 {
+					for _, extension := range extensionNamespaces {
+						extensions[extension.Name] = extension.Labels[k8s.LinkerdExtensionLabel]
+					}
+
+					// Retrieve all the extension names
+					extensionNames := make([]string, 0, len(extensions))
+					for _, v := range extensions {
+						extensionNames = append(extensionNames, fmt.Sprintf("* %s", v))
+					}
+
+					fmt.Fprintln(os.Stderr, fmt.Sprintf("Please uninstall the following extensions before uninstalling the control-plane:\n\t%s", strings.Join(extensionNames, "\n\t")))
+					fail = true
+				}
+
 				podList, err := k8sAPI.CoreV1().Pods("").List(cmd.Context(), metav1.ListOptions{LabelSelector: k8s.ControllerNSLabel})
 				if err != nil {
 					return err
@@ -41,14 +67,18 @@ This command provides all Kubernetes namespace-scoped and cluster-scoped resourc
 
 				var injectedPods []string
 				for _, pod := range podList.Items {
-					// skip core control-plane namespace
-					if pod.Namespace != controlPlaneNamespace {
+					// skip core control-plane namespace, and extension namespaces
+					if pod.Namespace != controlPlaneNamespace && extensions[pod.Namespace] == "" {
 						injectedPods = append(injectedPods, fmt.Sprintf("* %s", pod.Name))
 					}
 				}
 
 				if len(injectedPods) > 0 {
 					fmt.Fprintln(os.Stderr, fmt.Sprintf("Please uninject the following pods before uninstalling the control-plane:\n\t%s", strings.Join(injectedPods, "\n\t")))
+					fail = true
+				}
+
+				if fail {
 					os.Exit(1)
 				}
 			}