Primary VDI and Resize XenAPI tests interfering with each other

[Rick Harris]

The following two tests in test_xenapi are interfering with each other:

- test_instance_snapshot_fails_with_no_primary_vdi
- test_migrate_rollback_when_resize_down_fs_fails

When only those two tests are run, we get the following result:

http://paste.openstack.org/show/37312/

I'm not sure how this landed in trunk other than that Jenkins may have run the tests in a different order avoiding the failure. That said, I'm able to consistently reproduce that, so that's looking less and less likely.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/29346

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/29346
Committed: http://github.com/openstack/nova/commit/4e560d49d4a8d08341dae6df5e123e1be1b3c779
Submitter: Jenkins
Branch: master

commit 4e560d49d4a8d08341dae6df5e123e1be1b3c779
Author: Rick Harris <email address hidden>
Date: Thu May 16 08:02:21 2013 +0000

    Mox should cleanup before stubs

    Tests that define both Mox and Stubs may not receive a proper cleanup by the
    `TestCase` base class. The reason is that the order in which the Mox and Stubs
    are unset may not necessarily match-up with the order in which the tests
    called them. For example:

    Suppose we have a test that Stubs a method in its `setUp` and then Mox's that
    same method in the test body:

        Stub: Original-Func -> Stubbed-Func
        Mox: Stubbed-Func -> Mocked-Func

    Now when `TestCase` ultimately does its `cleanUp`, it will UnStub before
    UnMocking:

        UnStub: Stubbed-Func -> Original-Func
        UnMox: Mocked-Func -> Stubbed-Func

    Notice that generic cleanup has mistakingly left `Stubbed-Func` in place.

    Given that Stubs and Mox can be performed in any order, the upshot here is
    that the generic cleanup can't always be relied on.

    The proposed solution has two parts: a best-practice change and a small
    code-change.

    The code-change is to reverse the current order to UnMox before UnStubbing.
    While in theory the order is arbitrary, in practice, we have lots of Stubs in
    `setUp` methods already, so UnMox'ing first makes sense (and it fixes a
    current bug to boot!)

    The best-practice change is to recommend against stubbing and mocking the same
    function as well as to always place stubs before mocks when relying on the
    generic cleanup. If a developers needs to deviate from this, cleanup of stubs
    created after mocks are their responsibility.

    Fixes bug 1180671

    Change-Id: I42ab8b55026c0a133625a7cc81ed8b960e9d47d7