Fix Queue stat unavailable error seen during SNMP service start (sonic-net#238)

- What I did

Ideally SNMP service starts only after swss/syncd comes up.
But due to timing of bring up, It could happen that SNMP is trying to retrieve queue stat counter before it is update by syncd.
This is only seen once when the SNMP service comes up, as from the next iteration, syncd has updated the queue stats and it is available for SNMP to use.
ERR snmp#snmp-subagent [sonic_ax_impl] ERROR: No queue stat counters found in the Counter DB. SyncD database is incoherent.
This message is seen only once in all the cases observed, which means that once the counters are populated, snmp is able to retrieve the counters.
- How I did it
If counters are not found, return empty dicts since SNMP is just supposed to collect data and provide the data it has.

- How to verify it
Added unit-test.
If counters_db is not update, querying the QueueStats MIB should not return any output.

Author: SuvarnaMeenakshi

src/sonic_ax_impl/mibs/__init__.py

@@ -44,7 +44,6 @@
 
 redis_kwargs = {'unix_socket_path': '/var/run/redis/redis.sock'}
 
-
 def get_neigh_info(neigh_key):
     """
     split neigh_key string of the format:
@@ -455,9 +454,9 @@ def init_sync_d_queue_tables(db_conn):
     if not port_queues_map:
         logger.debug("Counters DB does not contain ports")
         return {}, {}, {}
-    elif not queue_stat_map:
-        logger.error("No queue stat counters found in the Counter DB. SyncD database is incoherent.")
-        raise RuntimeError('The queue_stat_map is not defined')
+    if not queue_stat_map:
+        logger.debug("No queue stat counters found in the Counter DB.")
+        return {}, {}, {}
 
     for queues in port_queue_list_map.values():
         queues.sort()

tests/test_mibs.py

@@ -3,6 +3,7 @@
 from unittest import TestCase
 
 import tests.mock_tables.dbconnector
+from sonic_ax_impl import mibs
 
 if sys.version_info.major == 3:
     from unittest import mock
@@ -50,3 +51,14 @@ def test_init_sync_d_interface_tables(self):
         self.assertTrue(if_alias_map == {})
         self.assertTrue(if_id_map == {})
         self.assertTrue(oid_name_map == {})
+
+    @mock.patch('swsssdk.dbconnector.SonicV2Connector.get_all', mock.MagicMock(return_value=({})))
+    def test_init_sync_d_queue_tables(self):
+        mock_queue_stat_map = {}
+        db_conn = Namespace.init_namespace_dbs()
+
+        port_queues_map, queue_stat_map, port_queue_list_map = \
+            Namespace.get_sync_d_from_all_namespace(mibs.init_sync_d_queue_tables, db_conn)
+        self.assertTrue(port_queues_map == {})
+        self.assertTrue(queue_stat_map == {})
+        self.assertTrue(port_queue_list_map == {})