[v15] Machine ID: Render kubernetes template without exec plugin when…

… using non-directory destination (#43656)

* Render kubernetes template without exec plugin when using non-directory destination

* Switch to info level for warning

* Update lib/tbot/config/template_kubernetes.go

Co-authored-by: Edoardo Spadolini <edoardo.spadolini@goteleport.com>

* Address review feedback

---------

Co-authored-by: Edoardo Spadolini <edoardo.spadolini@goteleport.com>

lib/tbot/bot/destination.go

@@ -18,7 +18,10 @@
 
 package bot
 
-import "context"
+import (
+	"context"
+	"fmt"
+)
 
 // Destination can persist renewable certificates.
 type Destination interface {
@@ -53,4 +56,8 @@ type Destination interface {
 	// MarshalYAML enables the yaml package to correctly marshal the Destination
 	// as YAML including the type header.
 	MarshalYAML() (interface{}, error)
+
+	// Stringer so that Destination's implements fmt.Stringer which allows for
+	// better logging.
+	fmt.Stringer
 }

lib/tbot/config/template_kubernetes.go

@@ -35,6 +35,7 @@ import (
 	"github.com/gravitational/teleport/lib/kube/kubeconfig"
 	"github.com/gravitational/teleport/lib/tbot/bot"
 	"github.com/gravitational/teleport/lib/tbot/identity"
+	logutils "github.com/gravitational/teleport/lib/utils/log"
 )
 
 const defaultKubeconfigPath = "kubeconfig.yaml"
@@ -201,6 +202,22 @@ func (t *templateKubernetes) render(
 		kubernetesClusterName: t.clusterName,
 	}
 
+	// In exec plugin mode, we write the credentials to disk and write a
+	// kubeconfig that execs `tbot` to load those credentials.
+
+	// We only support directory mode for this since the exec plugin needs
+	// to know the path to read the credentials from, and this is
+	// unpredictable with other types of destination.
+	destinationDir, isDirectoryDest := destination.(*DestinationDirectory)
+	if !t.disableExecPlugin {
+		if !isDirectoryDest {
+			log.InfoContext(
+				ctx,
+				"Kubernetes template will be rendered without exec plugin because destination is not a directory. Explicitly set `disable_exec_plugin: true` in the output to suppress this message",
+				"destination", logutils.StringerAttr(destination))
+			t.disableExecPlugin = true
+		}
+	}
 	var cfg *clientcmdapi.Config
 	if t.disableExecPlugin {
 		// If they've disabled the exec plugin, we just write the credentials
@@ -210,20 +227,6 @@ func (t *templateKubernetes) render(
 			return trace.Wrap(err)
 		}
 	} else {
-		// In exec plugin mode, we write the credentials to disk and write a
-		// kubeconfig that execs `tbot` to load those credentials.
-
-		// We only support directory mode for this since the exec plugin needs
-		// to know the path to read the credentials from, and this is
-		// unpredictable with other types of destination.
-		destinationDir, ok := destination.(*DestinationDirectory)
-		if !ok {
-			return trace.BadParameter(
-				"Destination %s must be a directory in exec plugin mode",
-				destination,
-			)
-		}
-
 		executablePath, err := t.executablePathGetter()
 		if err != nil {
 			return trace.Wrap(err)