[multi-asic]: Udpate to use SonicDBConfig from swsscommon (sonic-net#219)

**- What I did**
Update snmpagent to use SonicDBConfig from swsscommon.

**- How I did it**
- Update import of SonicDBConfig 
- init_namespace_dbs should return list of all namespace connector classes. Ensure that the list ordering is maintained such that the first element of the list is of the default namespace.
 
**- How to verify it**
Tested on single asic VS; SNMP service starts without any error log.
Execute snmpwalk on one of the OIDs:
admin@vlab-01:~$ docker exec -it snmp snmpwalk -v2c -c public 127.0.0.1 iso.3.6.1.2.1.2.2.1.2
iso.3.6.1.2.1.2.2.1.2.1 = STRING: "fortyGigE0/0"
iso.3.6.1.2.1.2.2.1.2.5 = STRING: "fortyGigE0/4"
iso.3.6.1.2.1.2.2.1.2.9 = STRING: "fortyGigE0/8"
iso.3.6.1.2.1.2.2.1.2.13 = STRING: "fortyGigE0/12"
iso.3.6.1.2.1.2.2.1.2.17 = STRING: "fortyGigE0/16"
iso.3.6.1.2.1.2.2.1.2.21 = STRING: "fortyGigE0/20"
iso.3.6.1.2.1.2.2.1.2.25 = STRING: "fortyGigE0/24"
iso.3.6.1.2.1.2.2.1.2.29 = STRING: "fortyGigE0/28"
..
iso.3.6.1.2.1.2.2.1.2.10000 = STRING: "eth0"

Tested on multi asic VS; SNMP service starts without any error log.
admin@vlab-08:~$ docker exec -it snmp snmpwalk -v2c -c public 127.0.0.1 iso.3.6.1.2.1.2.2.1.2
Execute snmpwalk on one of the OIDs:
iso.3.6.1.2.1.2.2.1.2.1 = STRING: "Ethernet1/1"
iso.3.6.1.2.1.2.2.1.2.5 = STRING: "Ethernet1/2"
iso.3.6.1.2.1.2.2.1.2.9 = STRING: "Ethernet1/3"
iso.3.6.1.2.1.2.2.1.2.13 = STRING: "Ethernet1/4"
iso.3.6.1.2.1.2.2.1.2.17 = STRING: "Ethernet1/5"
iso.3.6.1.2.1.2.2.1.2.21 = STRING: "Ethernet1/6"
iso.3.6.1.2.1.2.2.1.2.25 = STRING: "Ethernet1/7"
iso.3.6.1.2.1.2.2.1.2.29 = STRING: "Ethernet1/8"
iso.3.6.1.2.1.2.2.1.2.1001 = STRING: "PortChannel0001"
..
iso.3.6.1.2.1.2.2.1.2.9000 = STRING: "Eth4-ASIC0"
iso.3.6.1.2.1.2.2.1.2.9004 = STRING: "Eth5-ASIC0"
iso.3.6.1.2.1.2.2.1.2.9008 = STRING: "Eth6-ASIC0"
iso.3.6.1.2.1.2.2.1.2.9012 = STRING: "Eth7-ASIC0"
...

Author: SuvarnaMeenakshi

src/sonic_ax_impl/mibs/__init__.py

@@ -3,12 +3,13 @@
 import os
 
 from swsscommon.swsscommon import SonicV2Connector
-from swsssdk import SonicDBConfig
+from swsscommon.swsscommon import SonicDBConfig
 from swsssdk import port_util
 from swsssdk.port_util import get_index_from_str
 from ax_interface.mib import MIBUpdater
 from ax_interface.util import oid2tuple
 from sonic_ax_impl import logger
+from sonic_py_common import multi_asic
 
 COUNTERS_PORT_NAME_MAP = 'COUNTERS_PORT_NAME_MAP'
 COUNTERS_QUEUE_NAME_MAP = 'COUNTERS_QUEUE_NAME_MAP'
@@ -218,7 +219,12 @@ def init_db():
     Connects to DB
     :return: db_conn
     """
-    SonicDBConfig.load_sonic_global_db_config()
+    if not SonicDBConfig.isInit():
+        if multi_asic.is_multi_asic():
+            # Load the global config file database_global.json once.
+            SonicDBConfig.load_sonic_global_db_config()
+        else:
+            SonicDBConfig.load_sonic_db_config()
     # SyncD database connector. THIS MUST BE INITIALIZED ON A PER-THREAD BASIS.
     # Redis PubSub objects (such as those within swsssdk) are NOT thread-safe.
     db_conn = SonicV2Connector(**redis_kwargs)
@@ -538,10 +544,20 @@ class Namespace:
     @staticmethod
     def init_namespace_dbs():
         db_conn = []
-        SonicDBConfig.load_sonic_global_db_config()
-        for namespace in SonicDBConfig.get_ns_list():
-            db = SonicV2Connector(use_unix_socket_path=True, namespace=namespace, decode_responses=True)
+        if not SonicDBConfig.isInit():
+            if multi_asic.is_multi_asic():
+                SonicDBConfig.load_sonic_global_db_config()
+            else:
+                SonicDBConfig.load_sonic_db_config()
+        host_namespace_idx = 0
+        for idx, namespace in enumerate(SonicDBConfig.get_ns_list()): 
+            if namespace == multi_asic.DEFAULT_NAMESPACE:
+                host_namespace_idx = idx
+            db = SonicV2Connector(use_unix_socket_path=True, namespace=namespace)
             db_conn.append(db)
+        # Ensure that db connector of default namespace is the first element of
+        # db_conn list.
+        db_conn[0], db_conn[host_namespace_idx] = db_conn[host_namespace_idx], db_conn[0]
 
         Namespace.connect_namespace_dbs(db_conn)
         return db_conn

tests/mock_tables/dbconnector.py

@@ -9,6 +9,7 @@
 from swsssdk import SonicDBConfig
 from swsssdk.interface import DBInterface
 from swsscommon import swsscommon
+from sonic_py_common import multi_asic
 
 
 if sys.version_info >= (3, 0):
@@ -133,12 +134,12 @@ def keys(self, pattern='*'):
         # Find every key that matches the pattern
         return [key for key in self.redis.keys() if regex.match(key)]
 
-
 DBInterface._subscribe_keyspace_notification = _subscribe_keyspace_notification
 mockredis.MockRedis.config_set = config_set
 redis.StrictRedis = SwssSyncClient
 SonicV2Connector.connect = connect_SonicV2Connector
 swsscommon.SonicV2Connector = SonicV2Connector
+swsscommon.SonicDBConfig = SonicDBConfig
 
 # pytest case collecting will import some module before monkey patch, so reload
 from importlib import reload