XenAPI: cache_images without use_cow_images fails on multiple builds

[Rick Harris]

If you enable FLAGS.cache_images but not FLAGS.use_cow_images (a perfectly valid use-case), and then try to create multiple builds, the second build will error out with the following traceback:

 http://paste.openstack.org/show/18376/

The issue is that a VDI can only be copied one at a time, so we either have to re-download the image in this case, or we need to block until the image has been cached.

I'd vote that we bail out of the caching logic in this edge case and just re-download.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/8322

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/8322
Committed: http://github.com/openstack/nova/commit/93ca3da26bad785b1f30c3be5947de4f9f6d14af
Submitter: Jenkins
Branch: master

commit 93ca3da26bad785b1f30c3be5947de4f9f6d14af
Author: Rick Harris <email address hidden>
Date: Fri Jun 8 00:15:41 2012 +0000

    Avoid partially finished cache files.

    `VDI.copy` stores half-completed files directly in the SR where we may
    inadvertenly make use of them. To avoid these 'half-baked' records, we
    compare its current size to its expected size.

    Fixes bug 1009750

    Change-Id: I0f279ce69bfa0842a6bc74f19fd61462c6013b28