Comment 1 for bug 1571666

Looks like my initial analysis in not fully correct. So now it looks like this error is observed due to using nested transaction in allocate block of reference ipam driver [1]:

Resulting error:
2016-04-13 12:15:04.685 18292 ERROR oslo_db.sqlalchemy.exc_filters [req-15281429-cfa8-4b9f-a904-0105fc947933 admin -] DBAPIError exception wrapped from (pymysql.err.InternalError) (1305, u'SAVEPOINT sa_savepoint_3 does not exist') [SQL: u'ROLLBACK TO SAVEPOINT sa_savepoint_3']

Original error:
 <class 'oslo_db.exception.DBDeadlock'> (pymysql.err.InternalError) (1213, u'Deadlock found when trying to get lock; try restarting transaction') [SQL: u'INSERT INTO ipamallocations (ip_address, status, ipam_subnet_id) VALUES (%(ip_address)s, %(status)s, %(ipam_subnet_id)s)'] [parameters: {'status': 'ALLOCATED', 'ip_address': u'2001:db8::4e', 'ipam_subnet_id': u'1c084441-cec0-4865-848a-a75f01711e36'}]

When DBDeadlock is raised inside nested transaction block, nested transaction wrapper tries to rollback to safepoint where nested transaction was initiated. But looks like in DBDeadlock case this is not possible.

[1] https://github.com/openstack/neutron/blob/2dc9d67f6e0fbec8272dc1dfdb594d05378536d5/neutron/ipam/drivers/neutrondb_ipam/driver.py#L338