Cinder volume/snapshot cascade delete not successful

[databunnysg]

Issue:
when using cinder client with --cascade parameter to delete volume with snapshot attached
#cinder delete --cascade 2762015d-cad8-4b03-a117-63e10ea9406b
Request to delete volume 2762015d-cad8-4b03-a117-63e10ea9406b has been accepted.
Unexpected result: the snapshot and any volume cloned from this snapshot not deleted
Expected result: volume, snapshot and volume cloned from snapshot all deleted

This also failed tempest test case: https://docs.openstack.org/tempest/latest/_modules/volume/test_volume_delete_cascade.html

[YiHuangDB]

The root cause of this issue: OpenStack requires a snapshot to be independent of child volume. However, the existing truenas /zfs/snapshot/clone api does not support this.
For example, VolumeA  create snapsnot volumeA@snapshot,  volumeA@snapshot create child volume volumeAsnapshotchild. At this time, volumeA@snapshot should be able to removed safely. There should be no dependency between volumeA@snapshot and volumeAsnapshotchild. However this is not possible with the current Truenas/freenas /zfs/snapshot/clone api. Promote volumeAsnapshotchild won't help, cause as long as VolumeA exists there are dependencies prevented from volumeA@snapshot to be removed.