metropolis/cli/metroctl: refactor to use RunE instead of log.Fatal

Change-Id: Id5ca65980816e1715a8f08afcdf712292117012a
Reviewed-on: https://review.monogon.dev/c/monogon/+/3441
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/cli/metroctl/cmd_k8s_configure.go b/metropolis/cli/metroctl/cmd_k8s_configure.go
index cd90416..5ce4032 100644
--- a/metropolis/cli/metroctl/cmd_k8s_configure.go
+++ b/metropolis/cli/metroctl/cmd_k8s_configure.go
@@ -2,6 +2,7 @@
 
 import (
 	"context"
+	"fmt"
 	"log"
 	"os"
 	"os/exec"
@@ -24,36 +25,35 @@
 to connect to a Metropolis cluster. A cluster endpoint must be provided with the
 --endpoints parameter.`,
 	Args: PrintUsageOnWrongArgs(cobra.ExactArgs(0)),
-	Run:  doK8sConfigure,
-}
-
-func doK8sConfigure(cmd *cobra.Command, _ []string) {
-	ctx, _ := signal.NotifyContext(context.Background(), os.Interrupt)
-	if len(flags.clusterEndpoints) < 1 {
-		log.Fatalf("k8s configure requires at least one cluster endpoint to be provided with the --endpoints parameter.")
-	}
-
-	contextName, err := cmd.Flags().GetString("context")
-	if err != nil || contextName == "" {
-		log.Fatalf("k8s configure requires a valid context name to be provided with the --context parameter.")
-	}
-
-	// If the user has metroctl in their path, use the metroctl from path as
-	// a credential plugin. Otherwise use the path to the currently-running
-	// metroctl.
-	metroctlPath := "metroctl"
-	if _, err := exec.LookPath("metroctl"); err != nil {
-		metroctlPath, err = os.Executable()
-		if err != nil {
-			log.Fatalf("Failed to create kubectl entry as metroctl is neither in PATH nor can its absolute path be determined: %v", err)
+	RunE: func(cmd *cobra.Command, _ []string) error {
+		ctx, _ := signal.NotifyContext(context.Background(), os.Interrupt)
+		if len(flags.clusterEndpoints) < 1 {
+			return fmt.Errorf("k8s configure requires at least one cluster endpoint to be provided with the --endpoints parameter")
 		}
-	}
-	// TODO(q3k, issues/144): this only works as long as all nodes are kubernetes controller
-	// nodes. This won't be the case for too long. Figure this out.
-	if err := core.InstallKubeletConfig(ctx, metroctlPath, connectOptions(), contextName, flags.clusterEndpoints[0]); err != nil {
-		log.Fatalf("Failed to install metroctl/k8s integration: %v", err)
-	}
-	log.Printf("Success! kubeconfig is set up. You can now run kubectl --context=%s ... to access the Kubernetes cluster.", contextName)
+
+		contextName, err := cmd.Flags().GetString("context")
+		if err != nil || contextName == "" {
+			return fmt.Errorf("k8s configure requires a valid context name to be provided with the --context parameter")
+		}
+
+		// If the user has metroctl in their path, use the metroctl from path as
+		// a credential plugin. Otherwise use the path to the currently-running
+		// metroctl.
+		metroctlPath := "metroctl"
+		if _, err := exec.LookPath("metroctl"); err != nil {
+			metroctlPath, err = os.Executable()
+			if err != nil {
+				return fmt.Errorf("failed to create kubectl entry as metroctl is neither in PATH nor can its absolute path be determined: %w", err)
+			}
+		}
+		// TODO(q3k, issues/144): this only works as long as all nodes are kubernetes controller
+		// nodes. This won't be the case for too long. Figure this out.
+		if err := core.InstallKubeletConfig(ctx, metroctlPath, connectOptions(), contextName, flags.clusterEndpoints[0]); err != nil {
+			return fmt.Errorf("failed to install metroctl/k8s integration: %w", err)
+		}
+		log.Printf("Success! kubeconfig is set up. You can now run kubectl --context=%s ... to access the Kubernetes cluster.", contextName)
+		return nil
+	},
 }
 
 func init() {