[snmp] Allow system with no ports in config db run without errors (#221)

**What I did**
Allow system with no ports in config db run without errors.
This is needed for modular system which should boot properly without line cards.

**How I did it**
Remove snmpagent error exit if there are no ports in config DB or in counters DB.

**How to verify it**
Run snmpwalk on the root oid.

src/sonic_ax_impl/mibs/__init__.py

@@ -271,7 +271,7 @@ def init_sync_d_interface_tables(db_conn):
 
     # { if_name (SONiC) -> sai_id }
     # ex: { "Ethernet76" : "1000000000023" }
-    if_name_map_util, if_id_map_util = port_util.get_interface_oid_map(db_conn)
+    if_name_map_util, if_id_map_util = port_util.get_interface_oid_map(db_conn, blocking=False)
     for if_name, sai_id in if_name_map_util.items():
         if_name_str = if_name
         if (re.match(port_util.SONIC_ETHERNET_RE_PATTERN, if_name_str) or \
@@ -297,12 +297,8 @@ def init_sync_d_interface_tables(db_conn):
 
     # SyncD consistency checks.
     if not oid_name_map:
-        # In the event no interface exists that follows the SONiC pattern, no OIDs are able to be registered.
-        # A RuntimeError here will prevent the 'main' module from loading. (This is desirable.)
-        message = "No interfaces found matching pattern '{}'. SyncD database is incoherent." \
-            .format(port_util.SONIC_ETHERNET_RE_PATTERN)
-        logger.error(message)
-        raise RuntimeError(message)
+        logger.debug("There are no ports in counters DB")
+        return {}, {}, {}, {}
     elif len(if_id_map) < len(if_name_map) or len(oid_name_map) < len(if_name_map):
         # a length mismatch indicates a bad interface name
         logger.warning("SyncD database contains incoherent interface names. Interfaces must match pattern '{}'"
@@ -424,7 +420,7 @@ def init_sync_d_queue_tables(db_conn):
 
     # { Port name : Queue index (SONiC) -> sai_id }
     # ex: { "Ethernet0:2" : "1000000000023" }
-    queue_name_map = db_conn.get_all(COUNTERS_DB, COUNTERS_QUEUE_NAME_MAP, blocking=True)
+    queue_name_map = db_conn.get_all(COUNTERS_DB, COUNTERS_QUEUE_NAME_MAP, blocking=False)
     logger.debug("Queue name map:\n" + pprint.pformat(queue_name_map, indent=2))
 
     # Parse the queue_name_map and create the following maps:
@@ -455,10 +451,8 @@ def init_sync_d_queue_tables(db_conn):
 
     # SyncD consistency checks.
     if not port_queues_map:
-        # In the event no queue exists that follows the SONiC pattern, no OIDs are able to be registered.
-        # A RuntimeError here will prevent the 'main' module from loading. (This is desirable.)
-        logger.error("No queues found in the Counter DB. SyncD database is incoherent.")
-        raise RuntimeError('The port_queues_map is not defined')
+        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')

src/sonic_ax_impl/mibs/ieee802_1ab.py

@@ -196,7 +196,7 @@ def reinit_data(self):
             self.if_range.append((if_oid, ))
         self.if_range.sort()
         if not self.loc_port_data:
-            logger.warning("0 - b'PORT_TABLE' is empty. No local port information could be retrieved.")
+            logger.debug("0 - b'PORT_TABLE' is empty. No local port information could be retrieved.")
 
     def _get_if_entry(self, if_name):
         if_table = ""

tests/test_mibs.py

@@ -4,6 +4,11 @@
 
 import tests.mock_tables.dbconnector
 
+if sys.version_info.major == 3:
+    from unittest import mock
+else:
+    import mock
+
 modules_path = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 sys.path.insert(0, os.path.join(modules_path, 'src'))
 
@@ -32,3 +37,16 @@ def test_init_sync_d_lag_tables(self):
         self.assertTrue("PortChannel_Temp" in lag_name_if_name_map)
         self.assertTrue(lag_name_if_name_map["PortChannel_Temp"] == [])
         self.assertTrue(lag_sai_map["PortChannel01"] == "2000000000006")
+
+    @mock.patch('swsssdk.dbconnector.SonicV2Connector.get_all', mock.MagicMock(return_value=({})))
+    def test_init_sync_d_interface_tables(self):
+        db_conn = Namespace.init_namespace_dbs()
+
+        if_name_map, \
+        if_alias_map, \
+        if_id_map, \
+        oid_name_map = Namespace.get_sync_d_from_all_namespace(mibs.init_sync_d_interface_tables, db_conn)
+        self.assertTrue(if_name_map == {})
+        self.assertTrue(if_alias_map == {})
+        self.assertTrue(if_id_map == {})
+        self.assertTrue(oid_name_map == {})