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[Dave Walker <email@xxxxxxxxxx>]

** Also affects: neutron/kilo
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1501686

Title:
  Incorrect exception handling in DB code involving rollbacked
  transactions.

Status in neutron:
  Fix Released
Status in neutron kilo series:
  New

Bug description:
  I found out that some methods like  add_ha_port
  https://github.com/openstack/neutron/blob/master/neutron/db/l3_hamode_db.py#L312-L328
  contain the following logic:

  def create():
        create_something()
        try:
              _do_other_thing()
        except Exception:
               with excutils.save_and_reraise_exception():
                       delete_something()

  def  _do_other_thing():
       with context.session.begin(subtransactions=True):
           ....

  The problem is that when exception is raised in _do_other_thing it is
  caught in except block, but the object cannot be deleted in except
  section because internal transaction has been rolled back. We have
  tests on these methods, but they also are not correct (for example
  https://github.com/openstack/neutron/blob/master/neutron/tests/unit/db/test_l3_hamode_db.py#L360-L377)
  as methods  _do_other_thing() are mocked so inside transaction is
  never created and aborted.

  The possible solution is to use nested transaction in such places like
  this:

  def create()
         with context.session.begin(subtransactions=True):
                 create_something()
                 try:
                         _do_other_thing()
                 except Exception:
                         with excutils.save_and_reraise_exception():
                             delete_something()

  def _do_other_thing():
       with context.session.begin(nested=True):
           ....

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