Allow deleting tables before updating

Add table_delete to mod_config in the ConfigDBPipeConnector to allow table cleanup as part of transaction

Add unittest for ConfigDBConnector

setup.py

@@ -4,6 +4,7 @@
     'redis>=4.5.4;python_version >= "3.0"',
     'redis>=3.5.3;python_version < "3.0"',
     'redis-dump-load',
+    'fakeredis',
 ]
 
 high_performance_deps = [

src/swsssdk/configdb.py

@@ -392,19 +392,27 @@ def __mod_entry(self, pipe, table, key, data):
         else:
             pipe.hmset(_hash, self.typed_to_raw(data))
 
-    def mod_config(self, data):
+    def mod_config(self, data, table_delete=None):
         """Write multiple tables into config db.
            Extra entries/fields in the db which are not in the data are kept.
+           Pass one or more tables in table_delete to delete tables before update.
         Args:
             data: config data in a dictionary form
             { 
                 'TABLE_NAME': { 'row_key': {'column_key': 'value', ...}, ...},
                 'MULTI_KEY_TABLE_NAME': { ('l1_key', 'l2_key', ...) : {'column_key': 'value', ...}, ...},
                 ...
             }
+            table_delete: Table name(s) that will be deleted first before modify
         """
         client = self.get_redis_client(self.db_name)
         pipe = client.pipeline()
+        if table_delete:
+            if isinstance(table_delete, list):
+                for t in table_delete:
+                    self.__delete_table(client, pipe, t)
+            else:
+                self.__delete_table(client, pipe, table_delete)
         for table_name in data:
             table_data = data[table_name]
             if table_data is None:

test/test_configdb.py

@@ -0,0 +1,31 @@
+import os
+import sys
+
+from unittest import mock
+
+from unittest import TestCase
+
+modules_path = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+sys.path.insert(0, os.path.join(modules_path, 'src'))
+
+import swsssdk
+import fakeredis
+
+
+class TestConfigDB(TestCase):
+    def test__configdb_pipe(self):
+
+        s_db = swsssdk.ConfigDBPipeConnector()
+        #Use fakeredis to mock redis db
+        r_db = fakeredis.FakeStrictRedis(version=5)
+        s_db.get_redis_client = lambda db_name: r_db
+        s_db.db_name = "CONFIG_DB"
+
+        s_db.mod_config({'TABLE_NAME': { 'row_key': {'column_key1': 'valueA1', 'column_key2': 'valueB1'}}})
+        self.assertEqual(r_db.hget('TABLE_NAME|row_key', 'column_key1'), b'valueA1')
+        self.assertEqual(r_db.hget('TABLE_NAME|row_key', 'column_key2'), b'valueB1')
+        s_db.mod_config({'TABLE_NAME': { 'row_key': {'column_key1': 'valueA2'}}}, table_delete='TABLE_NAME')
+        self.assertEqual(r_db.hget('TABLE_NAME|row_key', 'column_key1'), b'valueA2')
+        self.assertEqual(r_db.hget('TABLE_NAME|row_key', 'column_key2'), None)
+
+