Comment 7 for bug 1296414

some discussion topics:

comments in _complete_deletion

        # FIXME(comstud): See bug 1296414. Quotas here should only be
        # committed if the instance is not SOFT_DELETED. If there were
        # some sort of race and the instance was SOFT_DELETED, we should
        # not commit quotas, as they would have already been done in
        # soft_delete().

I think we are not able to enter into this condition because _complete_deletion was called by _complete_partial_deletion and
_delete_instance

only if the instance state is DELETED you can enter _complete_partial_deletion
if we can call _complete_deletion in _delete_instance, that means instance state already become DELETED
(instance.vm_state = vm_states.DELETED)

also, if the _delete_instance was called with instance state SOFT_DELETED, the quotas.rollback() will be called
it will lead quotas.commit later take no effect since the quotas.reservations is already None

so I think it's not a problem