Attach a volume with scsi bus on an instance boot from cdrom fails

[Jose Castro Leon]

Following the upstream documentation to create images from an ISO (https://docs.openstack.org/nova/rocky/user/launch-instance-using-ISO-image.html) with an image from a vendor that required to have the disk attached on the scsi bus (/dev/sda) in order to install the system, it failed on attach volume step with the following traceback:

Driver failed to attach volume 95b42945-5d2a-4fba-b970-1d3fafcfbecc at /dev/sda: ValueError: max() arg is an empty sequence
Traceback (most recent call last):
   File "/usr/lib/python2.7/site-packages/nova/virt/block_device.py", line 538, in _volume_attach
     device_type=self['device_type'], encryption=encryption)
   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 1451, in attach_volume
     disk_info['unit'] = self._get_scsi_controller_max_unit(guest) + 1
   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 1134, in _get_scsi_controller_max_unit
     return max(ret)
 ValueError: max() arg is an empty sequence

Looking at the code

    def _get_scsi_controller_max_unit(self, guest):
        """Returns the max disk unit used by scsi controller"""
        xml = guest.get_xml_desc()
        tree = etree.fromstring(xml)
        addrs = "./devices/disk[@device='disk']/address[@type='drive']"

        ret = []
        for obj in tree.findall(addrs):
            ret.append(int(obj.get('unit', 0)))
        return max(ret)

As the instance has no existing devices in the scsi device bus, then the ret array is empty and the max function just fails with the traceback mentioned above.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/615141

[OpenStack Infra (hudson-openstack)]

Change abandoned by Lee Yarwood (<email address hidden>) on branch: master
Review: https://review.opendev.org/615141