Retry ebtables on race

[OpenStack Infra]

https://review.openstack.org/136217
commit fb9b2058051b771732f4425c97651128c8060441
Author: Chet Burgess <email address hidden>
Date: Thu Nov 20 18:29:15 2014 -0800

    Retry ebtables on race

    Calls to ebtables can race with libvirt and cause nova, or libvirt
    to fail to apply ebtables rules.

    The goal of this patch is to provide a simple fix to improve the
    stability of the gate.

    We now call ebtables in a simple loop that retries on failure.
    Long term we want to update nova to make use of the --concurrent
    flag in newer versions of ebtables. The --concurrent flag
    implements a lock to prevent multiple invocations of ebtables from
    racing. This will require a newer libvirt and the ability to
    timeout long running execs (--concurrent can block forever if it
    never gets the lock).

    A future patch is forthcoming to add support for --concurrent.

    DocImpact
    Add ebtables_exec_attempts option (default=3).

    Change-Id: I3e04782ac4678581462f9bee4bb10d5f3b223457
    Partial-Bug: #1316621

[Tom Fifield (fifieldt)]

http://docs.openstack.org/kilo/config-reference/content/nova-conf-changes-kilo.html