Comment 6 for bug 1443186

Reviewed: https://review.openstack.org/159275
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=d09785b97a282e8538642f6f8bcdd8491197ed74
Submitter: Jenkins
Branch: master

commit d09785b97a282e8538642f6f8bcdd8491197ed74
Author: Matt Riedemann <email address hidden>
Date: Wed Feb 25 14:13:45 2015 -0800

    Add config option to disable handling virt lifecycle events

    Historically the _sync_power_states periodic task has had the potential
    for race conditions and several changes have been made to try and
    tighten up this code:

    cc5388bbe81aba635fb757e202d860aeed98f3e8
    aa1792eb4c1d10e9a192142ce7e20d37871d916a
    baabab45e0ae0e9e35872cae77eb04bdb5ee0545
    bd8329b34098436d18441a8129f3f20af53c2b91

    The handle_lifecycle_events method which gets power state change events
    from the compute driver (currently only implemented by the libvirt
    driver) and calls _sync_instance_power_state - the same method that the
    _sync_power_states periodic task uses, except the periodic task at least
    locks when it's running - expands the scope for race problems in the
    compute manager so cloud providers should be able to turn it off. It is
    also known to have races with reboot where rebooted instances are
    automatically shutdown because of delayed lifecycle events that the
    instance is stopped even though it's running.

    This is consistent with the view that Nova should manage it's own state
    and not rely on external events telling it what to do about state
    changes. For example, in _sync_instance_power_state, if the Nova
    database thinks an instance is stopped but the hypervisor says it's
    running, the compute manager issues a force-stop on the instance.

    Also, although not documented (at least from what I can find), Nova has
    historically held a stance that it does not support out-of-band
    discovery and management of instances, so allowing external events to
    change state somewhat contradicts that stance and should be at least a
    configurable deployment option.

    DocImpact: New config option "handle_virt_lifecycle_events" in the
               DEFAULT group of nova.conf. By default the value is True
               so there is no upgrade impact or change in functionality.

    Related-Bug: #1293480
    Partial-Bug: #1443186
    Partial-Bug: #1444630

    Change-Id: I26a1bc70939fb40dc38e9c5c43bf58ed1378bcc7