Comment 4 for bug 1571666

Reviewed: https://review.openstack.org/306519
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=1f5008564de03bcd5d03743bdbce004bb27aaad1
Submitter: Jenkins
Branch: master

commit 1f5008564de03bcd5d03743bdbce004bb27aaad1
Author: Pavel Bondar <email address hidden>
Date: Fri Apr 15 18:29:19 2016 +0300

    Do not rewrite original exception for IPAM

    If deadlock exception is raised during allocation of multiple ips,
    this exception gets rewritten by rollback mechnism. Rollback tries
    to deallocate already allocated ips, but can't do it because database
    is not operatable unless entire transaction is restarted. So another
    exception is generated and it prevents retry wrappers from restarting
    transaction correctly.

    To fix this behavior rollback actions are covered with try-except block.
    This way any exception during rollback operations do not replace
    original exception.

    Partial-Bug: #1571666
    Change-Id: Ifdba7c49dc4a774be5221338b003d371750374d0