Add support for Downward API volumes (fieldRef in volume mounts)

[dciangot]

Feature Request: Downward API Volume Support

Currently, InterLink supports Downward API environment variables via fieldRef, but does not support Downward API volumes that expose pod metadata as files.

Current Support

✅ Environment variables via fieldRef:

env:
  - name: POD_NAME
    valueFrom:
      fieldRef:
        fieldPath: metadata.name

Missing Support

❌ Downward API volumes:

volumes:
  - name: podinfo
    downwardAPI:
      items:
        - path: "pod-name"
          fieldRef:
            fieldPath: metadata.name
        - path: "labels"
          fieldRef:
            fieldPath: metadata.labels
        - path: "annotations"
          fieldRef:
            fieldPath: metadata.annotations

Use Case

Downward API volumes are useful when:

• Applications need to read metadata from files rather than environment variables
• Multiple pieces of metadata need to be accessible in a structured way
• Labels and annotations need to be exposed (not supported in env fieldRef)
• Applications expect metadata in specific file locations

Expected Behavior

When a pod uses Downward API volumes, the virtual kubelet should:

1. Create the specified volume mount path in the container
2. Populate files with the requested pod metadata
3. Support all standard Downward API fieldRef paths in volumes:
   • metadata.name
   • metadata.namespace
   • metadata.uid
   • metadata.labels (volume-only feature)
   • metadata.annotations (volume-only feature)
   • spec.nodeName
   • spec.serviceAccountName
   • status.podIP
   • status.hostIP

Testing

Related test case development in: interlink-hq/vk-test-set#11

This would enable full Kubernetes Downward API compatibility in InterLink implementations.

[dciangot]

@Bianco95 is this done?