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[Lee Yarwood <1801342@xxxxxxxxxxxxxxxxxx>]

*** This bug is a duplicate of bug 1867075 ***
    https://bugs.launchpad.net/bugs/1867075

** This bug has been marked a duplicate of bug 1867075
   Arm64: Instance with Configure Drive attach volume failed failed

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https://bugs.launchpad.net/bugs/1801342

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

Status in OpenStack Compute (nova):
  In Progress

Bug description:
  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.

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