Unable to release finalizer for PVs created via legacy storage class

[rfranzke]

The out-of-tree CSI drivers are still (and forever?) compatible with the legacy (in-tree) provisioners, e.g. kubernetes.io/aws-ebs.
Provisioning + attaching a PV referencing such a storage class works perfectly fine with solely CSI. However, deleting such a volume is buggy as the external-attacher does not release its finalizer on the related PersistentVolume object.

It only watches for all VolumeAttachment and adds its finalizer to the referenced PersistentVolume on creation/update. However, when the PV gets deleted it checks whether .spec.csi is set which is not the case for a PV using a legacy provisioner. It correctly detaches the volume and releases the VolumeAttachment, though, the PersistentVolume will stuck forever in the system.

The culprit function seems to be

external-attacher/pkg/controller/controller.go

Lines 219 to 224 in 2d2ab15

	func (ctrl *CSIAttachController) processFinalizers(pv *v1.PersistentVolume) bool {
	if pv.Spec.CSI != nil && pv.Spec.CSI.Driver == ctrl.attacherName {
	return true
	}
	return false
	}

/kind bug

[rfranzke]

Steps for reproduction:

1. Create a Kubernetes cluster and install the necessary CSI controllers into it.
2. Create a StorageClass with a legacy provisioner:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: gp2
parameters:
  type: gp2
provisioner: kubernetes.io/aws-ebs
reclaimPolicy: Delete
volumeBindingMode: Immediate

3. Create a PersistentVolumeClaim referencing it and a Pod:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: ebs-claim3
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: gp2
  resources:
    requests:
      storage: 4Gi

---
apiVersion: v1
kind: Pod
metadata:
  name: app3
spec:
  containers:
  - name: app
    image: centos
    command: ["/bin/sh"]
    args: ["-c", "while true; do echo $(date -u) >> /data/out.txt; sleep 5; done"]
    volumeMounts:
    - name: persistent-storage
      mountPath: /data
  volumes:
  - name: persistent-storage
    persistentVolumeClaim:
      claimName: ebs-claim3

4. See the volume being correctly provisioned and attached. The external-attacher adds its finalizer correctly to both VolumeAttachment and PersistentVolume objects.
5. Delete the PVC and the pod again.
6. See the volume being correctly detached and deprovisioned/deleted from the infrastructure. However, observe that the the PersistentVolume is stuck in Terminating state as the finalizer of the external-attacher is still present.