Comment 2 for bug 1655979

Reviewed: https://review.openstack.org/419479
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=bff2030ecea8a1d21e03c61a7ece02f40dc25c5d
Submitter: Jenkins
Branch: master

commit bff2030ecea8a1d21e03c61a7ece02f40dc25c5d
Author: Stephen Finucane <email address hidden>
Date: Thu Jan 12 13:59:32 2017 +0000

    scheduler: Don't modify RequestSpec.numa_topology

    The 'NUMATopologyFilter' makes a call to 'numa_fit_instance_to_host' in
    order to determine whether an instance with a sample topology could fit
    on a given host. This function is provided with an InstanceNUMATopology
    object, which was extracted from the RequestSpec provided to the filter.
    However, the 'numa_fit_instance_to_host' call has the side effect of
    modifying a couple of fields on this InstanceNUMATopology object, most
    notably the pinning information, and these changes are then propagated
    to subsequent calls of the filter. The reason for this propagation is
    presumably Python's "call-by-object" model [1].

    While this doesn't cause any issues currently, it is a latent bug that
    has caused issues downstream. Resolve the issue by copying the entire
    RequestSpec object, thus ensuring any changes to this or the contained
    NUMA topology are not stored and cannot affect future calls to this or
    other filters.

    [1] https://jeffknupp.com/blog/2012/11/13/is-python-callbyvalue-or-callbyreference-neither/

    Change-Id: If26cbdd5189c53891554c8d128be9b90578616aa
    Closes-Bug: #1655979