[ModifyVolume] No VolumeModifyFailed event emitted when target VAC does not exist

[AndrewSirenko]

No VolumeModifyFailed nor VolumeModify event emitted when PVC is updated to point at non-existent VAC.

I deem this as an external-resizer bug because there is no easy way for customer to see why their volume is not modifying other than checking status.modifyVolumeStatus.status and seeing Pending.

Error message in resizer container:

csi-resizer E0729 00:03:26.778429 1 modify_volume.go:91] Get VAC with vac name <VAC-NAME> in VACInformer cache failed: volumeattributesclass.storage.k8s.io "io6-class" not found

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Reproduction steps:

1. Launch pod and associated PVC (with or without VolumeAttributeClassName pointing to existent VAC).
2. Patch PVC to non-existent VAC: kubectl patch pvc ebs-claim --patch '{"spec": {"volumeAttributesClassName": "io8-class"}}'
3. Describe PVC