Verify volume.GetPath() never returns ""

Add a new helper method volume.GetPath(Mounter) instead of calling
the GetPath() of the Mounter directly. Check if GetPath() is returning
a "" and convert that into an error. At this point, we only have
information about the type of the Mounter, so let's log that if
there is a problem

Fixes #23163

pkg/kubelet/kubelet.go

@@ -1012,7 +1012,11 @@ func (kl *Kubelet) relabelVolumes(pod *api.Pod, volumes kubecontainer.VolumeMap)
 	for _, vol := range volumes {
 		if vol.Mounter.GetAttributes().Managed && vol.Mounter.GetAttributes().SupportsSELinux {
 			// Relabel the volume and its content to match the 'Level' of the pod
-			err := filepath.Walk(vol.Mounter.GetPath(), func(path string, info os.FileInfo, err error) error {
+			path, err := volume.GetPath(vol.Mounter)
+			if err != nil {
+				return err
+			}
+			err = filepath.Walk(path, func(path string, info os.FileInfo, err error) error {
 				if err != nil {
 					return err
 				}
@@ -1053,7 +1057,10 @@ func makeMounts(pod *api.Pod, podDir string, container *api.Container, hostName,
 			vol.SELinuxLabeled = true
 			relabelVolume = true
 		}
-		hostPath := vol.Mounter.GetPath()
+		hostPath, err := volume.GetPath(vol.Mounter)
+		if err != nil {
+			return nil, err
+		}
 		if mount.SubPath != "" {
 			hostPath = filepath.Join(hostPath, mount.SubPath)
 		}

pkg/volume/util.go

@@ -18,6 +18,7 @@ package volume
 
 import (
 	"fmt"
+	"reflect"
 	"time"
 
 	"k8s.io/kubernetes/pkg/api"
@@ -193,6 +194,15 @@ func GenerateVolumeName(clusterName, pvName string, maxLength int) string {
 	return prefix + "-" + pvName
 }
 
+// Check if the path from the mounter is empty.
+func GetPath(mounter Mounter) (string, error) {
+	path := mounter.GetPath()
+	if path == "" {
+		return "", fmt.Errorf("Path is empty %s", reflect.TypeOf(mounter).String())
+	}
+	return path, nil
+}
+
 // ChooseZone implements our heuristics for choosing a zone for volume creation based on the volume name
 // Volumes are generally round-robin-ed across all active zones, using the hash of the PVC Name.
 // However, if the PVCName ends with `-<integer>`, we will hash the prefix, and then add the integer to the hash.