Utils.is_neutron can create race conditions

[Sean M. Collins]

It appears that using the utils.is_neutron() function can create race conditions, if not used carefully. Review #56381 was attempting to check the presence of Neutron, in order to determine if hairpinning should be enabled on the bridge that Nova creates. Only after creating a new configuration flag in Nova.conf and checking the setting, were the unit tests passing and the race condition resolved.

My hunch is that the global variable that utils.is_neutron( ) creates is the root cause of the issue.

https://review.openstack.org/#/c/56381/

[Matt Riedemann (mriedem)]

Remember that the tests are run in parallel, so that could have something to do with it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/72125

[Mark McLoughlin (markmc)]

I'm pretty confident this is just a case where some other test is calling is_neutron() with network_api_class set to neutron before this libvirt test is run

We should just call nova_utils.reset_is_neutron() in the libvirt test

I think is_neutron() can be tweaked to be safer ... but I don't think it was the cause of the problem in this case

[Mark McLoughlin (markmc)]

Reducing to low because I don't believe the unit test issues were caused by this

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/72125
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=2749bb9138f5946d1ef3030c7fe349fbd5920139
Submitter: Jenkins
Branch: master

commit 2749bb9138f5946d1ef3030c7fe349fbd5920139
Author: Russell Bryant <email address hidden>
Date: Sat Feb 8 16:42:08 2014 -0500

    Make is_neutron() thread-safe

    The utils method is_neutron() was not thread safe and could have returned the
    wrong value if two greenthreads called it at the same time. Specifically, a
    greenthread switch could occur when the import is being done. At that point, it
    looks like the check has already been completed, but the result has not yet been
    recorded.

    Closes-bug: #1274992
    Change-Id: I5a6eedaeb4c0deee4b31509946ae5daedf98389d