Use event.listen() instead of deprecated listeners kwarg

Fixes bug 1031405

The listeners kwarg is deprecated in SQLAlchemy 0.7.0. Since 0.7.3 is
the minimum required version of SQLAlchemy, use the new event.listen()
API instead.

Change-Id: I3d2bd2e65dbc6787d1c9b69ffd0bcff85c157869

nova/db/sqlalchemy/session.py

@@ -51,33 +51,27 @@ def get_session(autocommit=True, expire_on_commit=False):
     return session
 
 
-class SynchronousSwitchListener(sqlalchemy.interfaces.PoolListener):
-
+def synchronous_switch_listener(dbapi_conn, connection_rec):
     """Switch sqlite connections to non-synchronous mode"""
+    dbapi_conn.execute("PRAGMA synchronous = OFF")
 
-    def connect(self, dbapi_con, con_record):
-        dbapi_con.execute("PRAGMA synchronous = OFF")
-
-
-class MySQLPingListener(object):
 
+def ping_listener(dbapi_conn, connection_rec, connection_proxy):
     """
     Ensures that MySQL connections checked out of the
     pool are alive.
 
     Borrowed from:
     http://groups.google.com/group/sqlalchemy/msg/a4ce563d802c929f
     """
-
-    def checkout(self, dbapi_con, con_record, con_proxy):
-        try:
-            dbapi_con.cursor().execute('select 1')
-        except dbapi_con.OperationalError, ex:
-            if ex.args[0] in (2006, 2013, 2014, 2045, 2055):
-                LOG.warn('Got mysql server has gone away: %s', ex)
-                raise DisconnectionError("Database server went away")
-            else:
-                raise
+    try:
+        dbapi_conn.cursor().execute('select 1')
+    except dbapi_conn.OperationalError, ex:
+        if ex.args[0] in (2006, 2013, 2014, 2045, 2055):
+            LOG.warn('Got mysql server has gone away: %s', ex)
+            raise DisconnectionError("Database server went away")
+        else:
+            raise
 
 
 def is_db_connection_error(args):
@@ -116,13 +110,14 @@ def get_engine():
                 engine_args["poolclass"] = StaticPool
                 engine_args["connect_args"] = {'check_same_thread': False}
 
-            if not FLAGS.sqlite_synchronous:
-                engine_args["listeners"] = [SynchronousSwitchListener()]
+        _ENGINE = sqlalchemy.create_engine(FLAGS.sql_connection, **engine_args)
 
         if 'mysql' in connection_dict.drivername:
-            engine_args['listeners'] = [MySQLPingListener()]
-
-        _ENGINE = sqlalchemy.create_engine(FLAGS.sql_connection, **engine_args)
+            sqlalchemy.event.listen(_ENGINE, 'checkout', ping_listener)
+        elif "sqlite" in connection_dict.drivername:
+            if not FLAGS.sqlite_synchronous:
+                sqlalchemy.event.listen(_ENGINE, 'connect',
+                                        synchronous_switch_listener)
 
         if (FLAGS.sql_connection_trace and
                 _ENGINE.dialect.dbapi.__name__ == 'MySQLdb'):