Generate OVSDB schema helper in a separate method

Move the generation of the schema helper in OVSDB connection to a
different method

In the OVSDB connection class, move the code retrieving the schema
helper to a separate method. This will allow subclasses to override this
method, and retrieve the schema helper in a different manner, or provide
it to the Connection subclass in advance.

This use-case came up in Dragonflow, where we wanted to retrieve
specific columns within specific tables, and not the whole tables.

Change-Id: I634841a2402e5d9bcbf1983a17eee2bb1a6299fb
Related-Bug: 1612403

neutron/agent/ovsdb/native/connection.py

@@ -61,6 +61,7 @@ class Connection(object):
         self.lock = threading.Lock()
         self.schema_name = schema_name
         self.idl_class = idl_class or idl.Idl
+        self._schema_filter = None
 
     def start(self, table_name_list=None):
         """
@@ -70,30 +71,14 @@ class Connection(object):
                 schema_helper will register all tables for given schema_name as
                 default.
         """
+        self._schema_filter = table_name_list
         with self.lock:
             if self.idl is not None:
                 return
 
-            try:
-                helper = idlutils.get_schema_helper(self.connection,
-                                                    self.schema_name)
-            except Exception:
-                # We may have failed do to set-manager not being called
-                helpers.enable_connection_uri(self.connection)
+            helper = self.get_schema_helper()
+            self.update_schema_helper(helper)
 
-                # There is a small window for a race, so retry up to a second
-                @retrying.retry(wait_exponential_multiplier=10,
-                                stop_max_delay=1000)
-                def do_get_schema_helper():
-                    return idlutils.get_schema_helper(self.connection,
-                                                      self.schema_name)
-                helper = do_get_schema_helper()
-
-            if table_name_list is None:
-                helper.register_all()
-            else:
-                for table_name in table_name_list:
-                    helper.register_table(table_name)
             self.idl = self.idl_class(self.connection, helper)
             idlutils.wait_for_change(self.idl, self.timeout)
             self.poller = poller.Poller()
@@ -101,6 +86,32 @@ class Connection(object):
             self.thread.setDaemon(True)
             self.thread.start()
 
+    def get_schema_helper(self):
+        """Retrieve the schema helper object from OVSDB"""
+        try:
+            helper = idlutils.get_schema_helper(self.connection,
+                                                self.schema_name)
+        except Exception:
+            # We may have failed do to set-manager not being called
+            helpers.enable_connection_uri(self.connection)
+
+            # There is a small window for a race, so retry up to a second
+            @retrying.retry(wait_exponential_multiplier=10,
+                            stop_max_delay=1000)
+            def do_get_schema_helper():
+                return idlutils.get_schema_helper(self.connection,
+                                                  self.schema_name)
+            helper = do_get_schema_helper()
+
+        return helper
+
+    def update_schema_helper(self, helper):
+        if self._schema_filter:
+            for table_name in self._schema_filter:
+                helper.register_table(table_name)
+        else:
+            helper.register_all()
+
     def run(self):
         while True:
             self.idl.wait(self.poller)

neutron/tests/unit/agent/ovsdb/native/test_connection.py

@@ -50,6 +50,22 @@ class TestOVSNativeConnection(base.BaseTestCase):
     def test_start_with_table_name_list(self):
         self._test_start(table_name_list=['fake-table1', 'fake-table2'])
 
+    @mock.patch.object(connection, 'TransactionQueue')
+    @mock.patch.object(idl, 'Idl')
+    @mock.patch.object(idlutils, 'wait_for_change')
+    def test_start_call_graph(self, wait_for_change, idl, transaction_queue):
+        self.connection = connection.Connection(
+            mock.sentinel, mock.sentinel, mock.sentinel)
+        self.connection.get_schema_helper = mock.Mock()
+        helper = self.connection.get_schema_helper.return_value
+        self.connection.update_schema_helper = mock.Mock()
+        with mock.patch.object(poller, 'Poller') as poller_mock,\
+                mock.patch('threading.Thread'):
+            poller_mock.return_value.block.side_effect = eventlet.sleep
+            self.connection.start()
+        self.connection.get_schema_helper.assert_called_once_with()
+        self.connection.update_schema_helper.assert_called_once_with(helper)
+
     def test_transaction_queue_init(self):
         # a test to cover py34 failure during initialization (LP Bug #1580270)
         # make sure no ValueError: can't have unbuffered text I/O is raised