sqlalchemy session needs to be rolled back after catching a db exception

To avoid errors like this:

sqlalchemy.exc.InvalidRequestError: This Session's transaction has been rolled back by a nested rollback() call. To begin a new transaction, issue Session.rollback() first

the sqlalchemy session needs to be rolled back after catching a db exception in a transaction, see sqlalchemy faq http://docs.sqlalchemy.org/en/rel_0_8/faq.html#this-session-s-transaction-has-been-rolled-back-due-to-a-previous-exception-during-flush-or-similar . There are places in Neutron code where a db exception is caught and the session is not properly rolled back. As explained in the sqlalchemy faq, this is the right way:

try:
<use session>
session.commit()
except:
session.rollback()
raise
finally:
session.close() # optional, depends on use case