Comment 4 for bug 1527623

I think this might have been bad triage of the issue. The os-detach API in cinder is synchronous, the cinder-api service does a synchronous rpc call to cinder-volume to detach the volume and change the status to available:

https://github.com/openstack/cinder/blob/cbee6066e4ff92addf3452114f6f6be355a6ac40/cinder/volume/rpcapi.py#L217

https://github.com/openstack/cinder/blob/cbee6066e4ff92addf3452114f6f6be355a6ac40/cinder/volume/manager.py#L1363

I believe the problem in Tempest for TestVolumeBootPattern is that the volume has snapshots when tearing down which makes the volume detach in nova fail because the volume snapshots aren't deleted yet.