Comment 21 for bug 1792077

Reviewed: https://review.opendev.org/653510
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=7500c1910cf1cbe16ee48028b67aa919160f44da
Submitter: Zuul
Branch: stable/stein

commit 7500c1910cf1cbe16ee48028b67aa919160f44da
Author: melanie witt <email address hidden>
Date: Fri Oct 19 22:06:20 2018 +0000

    libvirt: set device address tag only if setting disk unit

    In Pike, we began setting disk unit values manually for the
    'virtio-scsi' controller model in order to allow up to 256 devices [1].
    We do this by setting the disk unit of the address tag manually for the
    guest config. If we do not set the address tag manually, libvirt would
    autogenerate it for us.

    A problem occurs when a user has a SCSI disk that is a volume or isn't
    using the 'virtio-scsi' controller model because we're not guarding our
    manual setting of the address tag in the guest config by the disk unit,
    in addition to the SCSI bus. This means that for a SCSI volume, we
    generate an address tag like '<address type="drive" controller="0"/>'
    for any SCSI volume, so a user with more than one device will get the
    following error when they try to boot an instance:

      Failed to start libvirt guest: libvirtError: unsupported
        configuration: Found duplicate drive address for disk with target name
        'sda' controller='0' bus='0' target='0' unit='0'

    This updates the conditionals to only manually set the address tag if
    the bus is SCSI _and_ the disk unit has been specified. Otherwise, let
    libvirt autogenerate the address tag and take care of avoiding
    collisions.

    [1] https://bugs.launchpad.net/nova/+bug/1686116

    Closes-Bug: #1792077

    Change-Id: Iefab05e84ccc0bf8f15bdbbf515a290d282dbc5d
    (cherry picked from commit 48fd81648a7cadf2d147a6beabac067c28b288b0)