Race when associating instance_uuid

[aeva black]

The association of a Nova instance to an Ironic node represents the "ownership" of that node. This is currently updated by sending a PATCH request to the API service, which will replace the current value of `nodes`.`instance_uuid` with the supplied value. Because this a node property, not a nested property, it is not possible to remove it; instance_uuid can only be replaced.

Even if the Nova driver implements a LBYL approach -- first, GET the node and check that instance_uuid is empty, then PATCH the new value -- it would still be possible for another client (another instance of nova-compute) to PATCH at the same time, leading to unexpected behavior.

Since Ironic does not return an exception when overwriting the instance_uuid, it is not currently possible for the Nova driver to implement a EAFP approach, and there is no way to avoid a race condition here.

[aeva black (tenbrae)]

For reference, I solved this in the nova baremetal code with the db.api method bm_node_associate_and_update, here:

http://git.openstack.org/cgit/openstack/nova/tree/nova/virt/baremetal/db/sqlalchemy/api.py#n190

[Haomeng,Wang (whaom)]

Devananda, ok I will try to port your code to our Ironic, and help me review, thanks.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/62160

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/62160
Committed: https://git.openstack.org/cgit/openstack/ironic/commit/?id=3f858cc6742078fa5c238f6e14dc443897abb7e6
Submitter: Jenkins
Branch: master

commit 3f858cc6742078fa5c238f6e14dc443897abb7e6
Author: Haomeng, Wang <email address hidden>
Date: Sat Dec 14 19:48:36 2013 +0800

    Avoid a race when associating instance_uuid

    Current code can potentially overwrite the instance_uuid without
    exception when multiple dbapi.update_node calls are issued. This
    patch adds a check to dbapi so that the NodeAssociated exception
    will be raised in this case.
    The Nova Ironic driver will handle this exception. This will be
    introduced in a separate patch.

    Change-Id: I28ef5ce55399bde7073ffc111e9bc4400e5577ee
    Partial-Bug: #1244541

[Haomeng,Wang (whaom)]

The dbapi code patch is merged, and for Ironic nova driver part will covered by this merged patch [1] to catch the exception.NodeAssociated to prevent duplicated instance_uuid population, add review comments already.

[1] https://review.openstack.org/62160
[2] https://review.openstack.org/#/c/51328/8/nova/virt/ironic/driver.py