Volume uuid of deleted volume is visible in volume list while restoring volume backup in horizon.

[Shubham Dang]

Issue:
When I deleted a volume and try to restore a backup of same volume which I deleted , the selected value in drop-down shows the UUID of volume which lead to problem.

Analysis:
When I checked the code and found that the value of volume UUID is picked from backup, but volume is not present anymore in the cloud.

The default value is set by this block of code: https://github.com/openstack/horizon/blob/2d2f944e2fe127433f2973ef77ba86ec997cf434/horizon/forms/fields.py#L303

the template used here for rendering: https://github.com/openstack/horizon/blob/master/horizon/templates/horizon/common/fields/_themable_select.html#L5

Code block responsible for choices of available volume:
https://github.com/openstack/horizon/blob/stable/train/openstack_dashboard/dashboards/project/backups/forms.py#L107

Steps to reproduce:
1. create a volume backup let's say vol
2. create backup 'backup' of volume 'vol'
3. delete the volume 'vol'
4. now restore backup 'back'

 A list of all available volume is shown, but the UUID of deleted volume 'vol' is also present in the list.

Actual:
UUID of deleted volume is visible in the list

Expected:
Volume UUID of deleted should not be present

Proposed Solution: If initial_value is not present in choices then take the first value of choices.

[Shubham Dang (shubhamdang7)]

Proposed Patch: https://review.opendev.org/c/openstack/horizon/+/792187

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/horizon 20.1.0 release.