Comment 7 for bug 1857139

Reviewed: https://review.opendev.org/700186
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=0d9622f581e830e7b7bc9763aaa09ba02e99b8bb
Submitter: Zuul
Branch: master

commit 0d9622f581e830e7b7bc9763aaa09ba02e99b8bb
Author: Matt Riedemann <email address hidden>
Date: Fri Dec 20 10:03:23 2019 -0500

    Handle cell failures in get_compute_nodes_by_host_or_node

    get_compute_nodes_by_host_or_node uses the scatter_gather_cells
    function but was not handling the case that a failure result
    was returned, which could be the called function raising some
    exception or the cell timing out. This causes issues when the
    caller of get_compute_nodes_by_host_or_node expects to get a
    ComputeNodeList back and can do something like len(nodes) on it
    which fails when the result is not iterable.

    To be clear, if a cell is down there are going to be problems
    which likely result in a NoValidHost error during scheduling, but
    this avoids an ugly TypeError traceback in the scheduler logs.

    Change-Id: Ia54b5adf0a125ae1f9b86887a07dd1d79821dd54
    Closes-Bug: #1857139