Comment 2 for bug 1821733

Reviewed: https://review.openstack.org/647831
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=50a0c6a4b98a3ef1d893b0bb204a4a4868017cbb
Submitter: Zuul
Branch: stable/queens

commit 50a0c6a4b98a3ef1d893b0bb204a4a4868017cbb
Author: Stephen Finucane <email address hidden>
Date: Tue Mar 26 13:32:56 2019 +0000

    [Stable Only] hardware: Handle races during pinning

    Due to how we do claiming of pinned CPUs and related NUMA "things", it's
    possible for claims to race. This raciness is usually not an issue since
    pinning with fail for the losing instance, which will just get
    rescheduled. This does mean that it's possible for an instance to land
    on a host with no CPUs at all though and this edge case is triggering a
    nasty bug made possible by Python's unusual scoping rules around for
    loops.

        >>> x = 5
        >>> for y in range(x):
        ... pass
        ...
        >>> print(y)
        4

    'y' would be considered out of scope in the above for most other
    languages (JS and its even dumber scoping rules aside, I guess) and it
    leaves us with situations where the variable might never exist, i.e. the
    bug at hand:

        >>> x = 0
        >>> for y in range(x):
        ... pass
        ...
        >>> print(y)
        Traceback (most recent call last):
        File "<stdin>", line 1, in <module>
        NameError: name 'y' is not defined

    Resolve this by adding a check to handle the "no CPUs at all" case and
    quick fail but also remove the reliance on this quirk of Python.

    This doesn't apply to stable/rocky since the issue was inadvertently
    resolved by changes I8982ab25338969cd98621f79b7fbec8af43d12c5 and
    I021ce59048d6292055af49457ba642022f648c81. However, those changes are
    significantly larger and backports have been previously rejected [1][2].

    [1] https://review.openstack.org/#/c/588570/
    [2] https://review.openstack.org/#/c/588571/

    Change-Id: I6afc3af9f13e3c1cc312112eb28eb6e10d2a9e07
    Signed-off-by: Stephen Finucane <email address hidden>
    Closes-Bug: #1821733