Fix snmp agent Initialize config DB multiple times issue (#245)

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**- What I did**
    Fix following code issue:
        1. When initialize SonicDBConfig on multi ASIC device, not check if the global config already initialized issue.
        2. Initialize SonicDBConfig multiple times with dupe code.

**- How I did it**
    Code change to check isGlobalInit before load global config.
    Move dupe code to Namespace.init_sonic_db_config() and initialize config only once.
    Add new mock method for SonicDBConfig.isGlobalInit

**- How to verify it**
    Pass all UT and E2E test.

**- Description for the changelog**
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Write a short (one line) summary that describes the changes in this
pull request for inclusion in the changelog:
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src/sonic_ax_impl/main.py

@@ -9,7 +9,7 @@
 import sys
 
 import ax_interface
-from sonic_ax_impl.mibs import ieee802_1ab
+from sonic_ax_impl.mibs import ieee802_1ab, Namespace
 from . import logger
 from .mibs.ietf import rfc1213, rfc2737, rfc2863, rfc3433, rfc4292, rfc4363
 from .mibs.vendor import dell, cisco
@@ -58,6 +58,8 @@ def main(update_frequency=None):
     global event_loop
 
     try:
+        Namespace.init_sonic_db_config()
+
         # initialize handler and set update frequency (or use the default)
         agent = ax_interface.Agent(SonicMIB, update_frequency or DEFAULT_UPDATE_FREQUENCY, event_loop)
 

src/sonic_ax_impl/mibs/__init__.py

@@ -218,16 +218,11 @@ def init_db():
     Connects to DB
     :return: db_conn
     """
-    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()
+    Namespace.init_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)
-
     return db_conn
 
 def init_mgmt_interface_tables(db_conn):
@@ -536,14 +531,32 @@ def get_oidvalue(self, oid):
         return self.oid_map[oid]
 
 class Namespace:
+
+    """
+        Sonic database initialized flag.
+    """
+    db_config_loaded = False
+
+    @staticmethod
+    def init_sonic_db_config():
+        """
+        Initialize SonicDBConfig
+        """
+        if Namespace.db_config_loaded:
+            return
+
+        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()
+
+        Namespace.db_config_loaded = True
+
     @staticmethod
     def init_namespace_dbs():
         db_conn = []
-        if not SonicDBConfig.isInit():
-            if multi_asic.is_multi_asic():
-                SonicDBConfig.load_sonic_global_db_config()
-            else:
-                SonicDBConfig.load_sonic_db_config()
+        Namespace.init_sonic_db_config()
         host_namespace_idx = 0
         for idx, namespace in enumerate(SonicDBConfig.get_ns_list()): 
             if namespace == multi_asic.DEFAULT_NAMESPACE:

tests/mock_tables/dbconnector.py

@@ -25,6 +25,9 @@ def clean_up_config():
     SonicDBConfig._sonic_db_global_config_init = False
     SonicDBConfig._sonic_db_config_init = False
 
+def mock_SonicDBConfig_isGlobalInit():
+    return SonicDBConfig._sonic_db_global_config_init
+
 
 # TODO Convert this to fixture as all Test classes require it.
 def load_namespace_config():
@@ -140,6 +143,7 @@ def keys(self, pattern='*'):
 SonicV2Connector.connect = connect_SonicV2Connector
 swsscommon.SonicV2Connector = SonicV2Connector
 swsscommon.SonicDBConfig = SonicDBConfig
+swsscommon.SonicDBConfig.isGlobalInit = mock_SonicDBConfig_isGlobalInit
 
 # pytest case collecting will import some module before monkey patch, so reload
 from importlib import reload