_snapshot_exists method in remotefs is wrong if the last snapshot was deleted by Nova

[Silvan Kaiser]

tl;dnr:
The _snapshot_exists method should return false if all snapshots have been deleted, even if the last snapshot was deleted via the Nova API.

Detailed:
Using the RemoteFSSnapDriverBase [1] implementation a volumes file name can change and thus cause _snapshot_exists to look for the wrong file, for example in the following scenario:
1) Create a volume and attach it to a VM
2) Create a snapshot file for this volume
3) Delete the snapshot file
4) Compare the volumes expected path to the path stored in the volumes .info file, they differ (which they should not).

The reason for this is that, when deleting the last snapshot of a volume via Nova API, the resulting file is not the original volume file but the file created during snapshotting, as Nova uses a pull operation (see [2]). This behaviour differs from the deletion of a snapshot of a detached volume [3], where the volumes file name stays the original file name.

This issue arises when the _snapshot_exists method [4] compares the active image path to the volumes path (which is based on the volume.name attribute, [5]). In case the last snapshot was deleted via Nova while the volume was attached this method reports remaining snapshots while there are none, as the volume.name attribute points to the original file name while the last active file name from the info file points to the new file name which was created during snapshotting.

[1] https://github.com/openstack/cinder/blob/master/cinder/volume/drivers/remotefs.py#L687
[2] https://github.com/openstack/cinder/blob/master/cinder/volume/drivers/remotefs.py#L1386
[3] https://github.com/openstack/cinder/blob/master/cinder/volume/drivers/remotefs.py#L1414
[4] https://github.com/openstack/cinder/blob/master/cinder/volume/drivers/remotefs.py#L1016
[5] https://github.com/openstack/cinder/blob/master/cinder/volume/drivers/remotefs.py#L442

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/632982

[Silvan Kaiser (2-silvan)]

Note: I updated the bug title and description. Initially i made a connection to the provider_location attribute which was wrong.