kubernetes: ConfigMap volume: do not mount empty dir when subPath is not found

What happened:

When a ConfigMap volume is marked as optional and the corresponding volumeMount references a specific key with subPath, kubelet mounts an empty directory whenever that key is missing.

This would be fine without the presence of the subPath, however the user expects a file and not a directory in the scenario described. I noticed the behaviour after seeing an app crash due to the presence of a directory where it expected either a file or nothing.

What you expected to happen:

kubelet figures out the key referenced by subPath does not exist and skips the volume when it creates the container.

How to reproduce it (as minimally and precisely as possible):

Create the following Pod:

apiVersion: v1
kind: Pod
metadata:
  name: empty-configmap-vol
spec:
  containers:
  - name: foo
    image: nginx:alpine
    volumeMounts:
    - mountPath: /conf/local.properties
      name: props
      subPath: my.file.content
  volumes:
  - name: props
    configMap:
      name: missing-configmap
      optional: true

An empty directory was mounted although the ConfigMap missing-configmap does not exist:

❯ kubectl exec empty-configmap-vol -- df -h
/dev/sda1                29.0G      4.8G     24.2G  17% /conf/local.properties

❯ kubectl exec empty-configmap-vol -- ls -l /conf
drwxrwxrwx    0 root     root             0 Oct 24 19:34 local.properties

Environment:

• Kubernetes version: 1.9.2
• Container runtime: Docker

/kind feature /sig node