Comment 11 for bug 1859766

Reviewed: https://review.opendev.org/703103
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=b874c409c11b5d83508d2f0276a9a648f72192a4
Submitter: Zuul
Branch: stable/stein

commit b874c409c11b5d83508d2f0276a9a648f72192a4
Author: Balazs Gibizer <email address hidden>
Date: Wed May 1 23:38:40 2019 +0200

    Enhance service restart in functional env

    Bugfix Icaf1bae8cb040b939f916a19ce026031ddb84af7 showed that restarting
    a compute service in the functional env is unrealistic causing faults
    to slip through. During that bug fix only the minimal change was done
    in the functional env regarding compute service restart to reproduce
    the reported fault. However the restart of the compute service could
    be made even more realistic.

    This patch simulates a compute service restart in the functional env
    by stopping the original compute service and starting a totally new
    compute service for the same host and node. This way we can make sure
    that we get a brand new ComputeManager in the new service and no
    state can leak between the old and the new service.

    This change revealed another shortcoming of the functional env.
    In the real world the nova-compute service could be restarted without
    loosing any running servers on the compute host. But with the naive
    implementation of this change the compute service is re-created. This
    means that a new ComputeManager is instantiated that loads a new
    FakeDriver instance as well. That new FakeDriver instance then reports
    an empty hypervisor. This behavior is not totally unrealistic as it
    simulates such a compute host restart that cleans the hypervisor state
    as well (e.g. compute host redeployment). However this type of restart
    shows another bug in the code path that destroys and deallocates
    evacuated instance from the source host. Therefore this patch
    implements the compute service restart in a way that simulates only a
    service restart and not a full compute restart. A subsequent patch will
    add a test that uses the clean hypervisor case to reproduces the
    revealed bug.

    Related-Bug: #1724172

    On stable/stein:

    Closes-Bug: #1859766

    Note: mock package import added to nova/test.py (due to not having patch
    Ibe7cb29620f06d31059f2a5f94ca180b8671046e in stable/stein)

    Change-Id: I9d6cd6259659a35383c0c9c21db72a9434ba86b1
    (cherry picked from commit 2794748d9c58623045023f34c7793c58ce41447c)