add an option to let the user choose the right time to start connection

Sometimes we don't want to start connection when the IDL is created,
because in some cases our resources may not be ready. so give users a
choice when to connect is better.

Change-Id: Iccf4c604127568719e4ebde60959f3b4ec04de58

ovsdbapp/backend/ovs_idl/__init__.py

@@ -26,9 +26,10 @@ class Backend(object):
     lookup_table = {}
     ovsdb_connection = None
 
-    def __init__(self, connection, **kwargs):
+    def __init__(self, connection, start=True, **kwargs):
         super(Backend, self).__init__(**kwargs)
-        self.start_connection(connection)
+        if start:
+            self.start_connection(connection)
 
     @classmethod
     def start_connection(cls, connection):

ovsdbapp/tests/unit/schema/open_vswitch/test_impl_idl.py

@@ -36,7 +36,16 @@ class TransactionTestCase(base.TestCase):
 
 
 class TestOvsdbIdl(base.TestCase):
+    def setUp(self):
+        super(TestOvsdbIdl, self).setUp()
+        impl_idl.OvsdbIdl.ovsdb_connection = None
+
     def test_nested_txns(self):
         conn = mock.MagicMock()
         api = impl_idl.OvsdbIdl(conn, nested_transactions=False)
         self.assertFalse(api._nested_txns)
+
+    def test_init_session(self):
+        conn = mock.MagicMock()
+        backend = impl_idl.OvsdbIdl(conn, start=False)
+        self.assertIsNone(backend.ovsdb_connection)