libvirt: the None value for the 'os_type' property is passed to glance when doing a snapshot

[Andrea Rosa]

This change: https://review.openstack.org/42877 introduced the ability to pass os_type property to glance when we are doing a snapshot, but the change doesn't manage correctly the None value for this property.

The result of that is that we have a porperty called 'os_type': None associated with the snapshotted image.
That is a problem as this property is used when we create the backing file for the ephemeral disks.

From nova/virt/libvirt/driver/py:
 # Lookup the filesystem type if required
 os_type_with_default = instance['os_type']
 if not os_type_with_default:
     os_type_with_default = 'default'

and then we create the ephemeral file name using this rule:

fname = "ephemeral_%s_%s" % (ephemeral_gb, os_type_with_default)

Now consider an instance with a flavor with an ephemeral disk of 50 G and without the os_type defined, the resulting backing file will be:
"ephemeral_50_default"
At this point if we create a snapshot from that instance and we boot from it we will have another backing file created and called:
"ephemeral_50_None"

That is bad for at least two reasons:
1 performance issue: we need to create a new backing file which is not actually necessary
2 resources leaking: those backing files consume disk capacity on the host

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/52138

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/52138
Committed: http://github.com/openstack/nova/commit/db0daa3455f114e90cefbe02e205ca1d327fc45c
Submitter: Jenkins
Branch: master

commit db0daa3455f114e90cefbe02e205ca1d327fc45c
Author: Andrea Rosa <email address hidden>
Date: Wed Oct 16 15:38:27 2013 +0100

    Manage None value for the 'os_type' property

    The I2bab617f6c1e8866650ca7dc5178b640738c9ae6 introduced a way to pass the
    'os_type' property to glance when we are doing a snapshot in the libvirt
    driver. Currently the code doesn't manage properly the None value for this
    property and so None value is passed to glance which creates an image with
    a property called 'os_type' with value 'None'.
    As results of that all instances booted from a snapshot with this property will
    have None value in the os_type field.
    That field is used to create file name for backing images for ephemeral disks.
    Now consider this scenario:
    we boot an instance from an image without the os_type property defined and
    with an ephemeral disk of 50 G
    the resulting backing file will be called ephemeral_50_default.
    At this point we take a snapshot from that instance and we boot a new instance
    using this snapshot, and because of this bug we will have a new backing file
    called ephemeral_50_None.
    That is bad because is leaking resources in the host.

    Please note that I did a small refactor of the code to make my change testable.

    Change-Id: I40bca2732ee2c911344e9d361612007da1c7c3cf
    Closes-bug: #1240542