Add ephemeral volume type for SnapshotPolicy sources

[kasefuchs]

What problem are you trying to solve?

I need to backup external data (for example an S3 bucket) using runJob hooks.

Right now SnapshotPolicy only accepts existing PVCs in spec.sources, so I have to create a temporary PVC just for the backup. After that it is no longer needed, but it still has to be managed and cleaned up.

Proposed solution

Add an ephemeral source type to spec.sources.

Kopiur could create a temporary PVC before the Snapshot, mount it into the backup hooks, and remove it after the Snapshot.

Example:

apiVersion: kopiur.home-operations.com/v1alpha1
kind: SnapshotPolicy
metadata:
  name: nextcloud-garage
spec:
  repository:
    kind: ClusterRepository
    name: shared
  credentialProjection:
    enabled: true
  copyMethod: Direct
  sources:
    - ephemeral:
        size: 50Gi
  hooks:
    beforeSnapshot:
      - runJob:
          jobSpec:
            template:
              spec:
                restartPolicy: Never
                containers:
                  - name: backup
                    image: amazon/aws-cli:latest
                    envFrom:
                      - secretRef:
                          name: nextcloud-garage-key
                    command:
                      - aws
                      - s3
                      - sync
                      - s3://nextcloud
                      - /backup
                    volumeMounts:
                      - name: backup
                        mountPath: /backup
                volumes:
                  - name: backup
                    persistentVolumeClaim:
                      claimName: <generated pvc name?>
  retention:
    keepLatest: 3
    keepDaily: 7
    keepWeekly: 2
  compression:
    compressor: zstd

Alternatives considered

Using a regular PVC and cleaning it up in an afterSnapshot hook.

This works, but requires creating and managing a PVC that is only needed for a single snapshot.

[perfectra1n]

I'm gonna be honest, I think that this is out of the scope of Kopiur...a lot of information is tied to the name of the PVC in the Snapshots

But this is also equally melting my brain to even stare at