Comment 8 for bug 1742467

Reviewed: https://review.openstack.org/615677
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=deef31729bd54f3747b7adba4132f148559c2242
Submitter: Zuul
Branch: master

commit deef31729bd54f3747b7adba4132f148559c2242
Author: Eric Fried <email address hidden>
Date: Mon Nov 5 16:04:10 2018 -0600

    Reduce calls to placement from _ensure

    Prior to this patch, the report client's update_from_provider_tree
    method would, upon failure of any placement API call, invalidate the
    cache *just* for the failing provider (and any descendants) and attempt
    to continue operating on any other providers in the tree.

    With this patch, we instead invalidate the tree around the failing
    provider and fail right away.

    In real life, since we don't yet have any implementations of nested,
    this would have been effectively a null change.

    Except: this allows us to resolve a TODO whereby we would *always*
    _ensure_resource_provider (including a call to GET
    /resource_providers?in_tree=$compute_rp) on every periodic. Now we can
    optimize that out.

    This should reduce the number of calls to placement per RT periodic to
    zero in steady state when [compute]resource_provider_association_refresh
    is zero.

    Closes-Bug: #1742467

    Change-Id: Ieeaad9783e0ff93377fbc6c7932618d2fac8946a