Comment 22 for bug 1670627

Reviewed: https://review.openstack.org/445236
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=f8c2df78f2ff8b58d72e55391b02ae8c6faa4bc1
Submitter: Jenkins
Branch: stable/ocata

commit f8c2df78f2ff8b58d72e55391b02ae8c6faa4bc1
Author: Matt Riedemann <email address hidden>
Date: Wed Mar 8 21:51:07 2017 -0500

    Decrement quota usage when deleting an instance in cell0

    When we fail to schedule an instance, e.g. there are no hosts
    available, conductor creates the instance in the cell0 database
    and deletes the build request. At this point quota usage
    has been incremented in the main 'nova' database.

    When the instance is deleted, the build request is already gone
    so _delete_while_booting returns False and we lookup the instance
    in cell0 and delete it from there, but that flow wasn't decrementing
    quota usage like _delete_while_booting was.

    This change adds the same quota usage decrement handling that
    _delete_while_booting performs.

    NOTE(mriedem): This change also pulls in some things from
    I7de87dce216835729283bca69f0eff59a679b624 which is not being
    backported to Ocata since in Pike it solves a slightly different
    part of this quota usage issue. In Pike the cell mapping db_connection
    is actually stored on the context object when we get the instance
    from nova.compute.api.API.get(). So the fix in Pike is slightly
    different from Ocata. However, what we need to pull from that Pike
    change is:

    1. We need to target the cell that the instance lives in to get the
       flavor information when creating the quota reservation.

    2. We need to change the functional regression test to assert that
       the bug is fixed.

    The code and tests are adjusted to be a sort of mix between both
    changes in Pike without requiring a full backport of the 2nd
    part of the fix in Pike.

    Change-Id: I4cb0169ce0de537804ab9129bc671d75ce5f7953
    Partial-Bug: #1670627
    (cherry picked from commit 018068c4caac324643c7c6a4360fad855dd096eb)