Comment 12 for bug 1403916

Reviewed: https://review.openstack.org/161328
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=8940579083bf5b572f88bce7d64a848ed823f950
Submitter: Jenkins
Branch: stable/juno

commit 8940579083bf5b572f88bce7d64a848ed823f950
Author: Mitsuhiro Tanino <email address hidden>
Date: Mon Feb 9 17:18:31 2015 -0500

    Clear migration_status from a destination volume if migration fails

    When some error happens during volume migration, cinder-volume
    keeps remaining a destination volume because the volume copy may be
    in the completing phase and a source volume may be deleted already.

    In this case, the migration_status of the destination volume in
    data base should be cleared. As a result, the volume can be handled
    properly even if volume migration is failed. If the migration is
    in the migrating phase, the volume should be deleted and an entry
    of database is also destroyed because the source volume is remaining.

    Also there is an another problem that a source volume is remaining
    as migrating status if create_export raises exception. In this case,
    source volume can't be deleted even if it will be unnecessary.

    This change fixes these two problems.

    NOTE:
    In order to backport test cases, we need to use contextlib.nested
    instead of using multiple variables in "with" statement because
    it is not supported at python2.6.

    Closes-Bug: #1403916
    Change-Id: Ie4199bc1e804e5cbbb793f448a99487f9823f1a9
    (cherry picked from commit d96c40c0ab85090a83cb532090cc879c27d630d5)