handle_dbapi_exception is missed on _revalidate_connection

this might be too risky for 0.9, here's a stack trace under 0.9.8 (the second one):
#!

(py27)vsergeev@vsergeev-pc:~/sandbox/nova$ python
Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from nova.db.sqlalchemy import api
>>> conn = api.get_engine().connect()
No handlers could be found for logger "oslo.db.sqlalchemy.session"
>>> conn.execute("select 1").scalar()
1L
>>> # stop mysql
... 
>>> conn.execute("select 1").scalar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 721, in execute
    return self._execute_text(object, multiparams, params)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 870, in _execute_text
    statement, parameters
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 958, in _execute_context
    context)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1155, in _handle_dbapi_exception
    if newraise:
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 951, in _execute_context
    context)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 436, in do_execute
    cursor.execute(statement, parameters)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/MySQLdb/cursors.py", line 205, in execute
    self.errorhandler(self, exc, value)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
oslo.db.exception.DBConnectionError: (OperationalError) (2006, 'MySQL server has gone away') 'select 1' ()
>>> conn.execute("select 1").scalar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 721, in execute
    return self._execute_text(object, multiparams, params)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 870, in _execute_text
    statement, parameters
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 893, in _execute_context
    None, None)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1155, in _handle_dbapi_exception
    if newraise:
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 236, in connection
    return self._revalidate_connection()
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 244, in _revalidate_connection
    self.__connection = self.engine.raw_connection()
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1848, in raw_connection
    return self.pool.unique_connection()
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 280, in unique_connection
    return _ConnectionFairy._checkout(self)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 644, in _checkout
    fairy = _ConnectionRecord.checkout(pool)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 442, in checkout
    dbapi_connection = rec.get_connection()
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 506, in get_connection
    self.connection = self.__connect()
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/pool.py", line 538, in __connect
    connection = self.__pool._creator()
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/strategies.py", line 96, in connect
    connection_invalidated=invalidated
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/strategies.py", line 90, in connect
    return dialect.connect(*cargs, **cparams)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 377, in connect
    return self.dbapi.connect(*cargs, **cparams)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/MySQLdb/__init__.py", line 81, in Connect
    return Connection(*args, **kwargs)
  File "/home/vsergeev/sandbox/nova/.tox/py27/local/lib/python2.7/site-packages/MySQLdb/connections.py", line 193, in __init__
    super(Connection, self).__init__(*args, **kwargs2)
sqlalchemy.exc.OperationalError: (OperationalError) (2002, "Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)") None None
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