[db_migrator] Migrate DNS configuratuion (sonic-net#2893)

What I did
sonic-net/sonic-buildimage#14549
This PR has introduced DNS configuration to CONFIG_DB, we need to migrate original CONFIG_DB to support DNS configuration during upgrading SONiC.

How I did it
If there's no DNS_NAMESERVER table in CONFIG_DB, read DNS_NAMESERVER table from minigraph and update CONFIG_DB.

How to verify it
Run unit test for db migrator.

scripts/db_migrator.py

@@ -47,7 +47,7 @@ def __init__(self, namespace, socket=None):
                      none-zero values.
               build: sequentially increase within a minor version domain.
         """
-        self.CURRENT_VERSION = 'version_4_0_3'
+        self.CURRENT_VERSION = 'version_4_0_4'
 
         self.TABLE_NAME      = 'VERSIONS'
         self.TABLE_KEY       = 'DATABASE'
@@ -637,6 +637,18 @@ def migrate_route_table(self):
             if 'protocol' not in route_attr:
                 self.appDB.set(self.appDB.APPL_DB, route_key, 'protocol', '')
 
+    def migrate_dns_nameserver(self):
+        """
+        Handle DNS_NAMESERVER table migration. Migrations handled:
+        If there's no DNS_NAMESERVER in config_DB, load DNS_NAMESERVER from minigraph
+        """
+        if not self.minigraph_data or 'DNS_NAMESERVER' not in self.minigraph_data:
+            return
+        dns_table = self.configDB.get_table('DNS_NAMESERVER')
+        if not dns_table:
+            for addr, config in self.minigraph_data['DNS_NAMESERVER'].items():
+                self.configDB.set_entry('DNS_NAMESERVER', addr, config)
+
     def migrate_routing_config_mode(self):
         # DEVICE_METADATA - synchronous_mode entry
         if not self.minigraph_data or 'DEVICE_METADATA' not in self.minigraph_data:
@@ -1020,9 +1032,19 @@ def version_4_0_2(self):
     def version_4_0_3(self):
         """
         Version 4_0_3.
-        This is the latest version for master branch
         """
         log.log_info('Handling version_4_0_3')
+        self.set_version('version_4_0_4')
+        return 'version_4_0_4'
+
+    def version_4_0_4(self):
+        """
+        Version 4_0_4.
+        This is the latest version for master branch
+        """
+        log.log_info('Handling version_4_0_4')
+        # Updating DNS nameserver
+        self.migrate_dns_nameserver()
         return None
 
     def get_version(self):

tests/db_migrator_input/config_db/dns-nameserver-expected.json

@@ -0,0 +1,11 @@
+{
+    "DNS_NAMESERVER|1.1.1.1": {
+        "state": "enabled"
+    },
+    "DNS_NAMESERVER|2001:1001:110:1001::1": {
+        "state": "enabled"
+    },
+    "VERSIONS|DATABASE": {
+        "VERSION": "version_4_0_4"
+    }
+}

tests/db_migrator_input/config_db/dns-nameserver-input.json

@@ -0,0 +1,5 @@
+{
+    "VERSIONS|DATABASE": {
+        "VERSION": "version_4_0_3"
+    }
+}

tests/db_migrator_test.py

@@ -294,6 +294,37 @@ def test_lacp_key_migrator(self):
         assert dbmgtr.configDB.get_table('PORTCHANNEL') == expected_db.cfgdb.get_table('PORTCHANNEL')
         assert dbmgtr.configDB.get_table('VERSIONS') == expected_db.cfgdb.get_table('VERSIONS')
 
+class TestDnsNameserverMigrator(object):
+    @classmethod
+    def setup_class(cls):
+        os.environ['UTILITIES_UNIT_TESTING'] = "2"
+
+    @classmethod
+    def teardown_class(cls):
+        os.environ['UTILITIES_UNIT_TESTING'] = "0"
+        dbconnector.dedicated_dbs['CONFIG_DB'] = None
+
+    def test_dns_nameserver_migrator(self):
+        dbconnector.dedicated_dbs['CONFIG_DB'] = os.path.join(mock_db_path, 'config_db', 'dns-nameserver-input')
+        import db_migrator
+        dbmgtr = db_migrator.DBMigrator(None)
+        # Set minigraph_data to DNS_NAMESERVERS
+        dbmgtr.minigraph_data = {
+            'DNS_NAMESERVER': {
+                '1.1.1.1': {},
+                '2001:1001:110:1001::1': {}
+            }
+        }
+        dbmgtr.migrate()
+        dbconnector.dedicated_dbs['CONFIG_DB'] = os.path.join(mock_db_path, 'config_db', 'dns-nameserver-expected')
+        expected_db = Db()
+        advance_version_for_expected_database(dbmgtr.configDB, expected_db.cfgdb, 'version_4_0_4')
+        resulting_keys = dbmgtr.configDB.keys(dbmgtr.configDB.CONFIG_DB, 'DNS_NAMESERVER*')
+        expected_keys = expected_db.cfgdb.keys(expected_db.cfgdb.CONFIG_DB, 'DNS_NAMESERVER*')
+
+        diff = DeepDiff(resulting_keys, expected_keys, ignore_order=True)
+        assert not diff
+
 class TestQosDBFieldValueReferenceRemoveMigrator(object):
     @classmethod
     def setup_class(cls):
@@ -681,5 +712,6 @@ def test_fast_reboot_upgrade_to_4_0_3(self):
         dbmgtr = db_migrator.DBMigrator(None)
         dbmgtr.migrate()
         expected_db = self.mock_dedicated_config_db(db_after_migrate)
+        advance_version_for_expected_database(dbmgtr.configDB, expected_db.cfgdb, 'version_4_0_3')
         assert not self.check_config_db(dbmgtr.configDB, expected_db.cfgdb)
         assert dbmgtr.CURRENT_VERSION == expected_db.cfgdb.get_entry('VERSIONS', 'DATABASE')['VERSION']