[ciscoPfcExtMIB]: Remove returning first intf index if subid is empty (…

…#322)

- What I did
When querying for PFC counters MIB cpfcIfTable if there is no interface present on the device, then there is an exception while handing get_next as the implementation tries to return the first index of an empty list.
This happens in case of VoQ chassis Supervisor where there are no interfaces present.
Anytime a snmp query is made to cpfcIfTable 1.3.6.1.4.1.9.9.813.1.1, the exception message gets logged on the device.

- How I did it
Currently if sub_id is None, if_range[0] is returned. IF if_range is empty array this can cause an Exception.
Removed this check to avoid returning if_range[0].
The check "if not sub_id" is not required here.
right = bisect_right(self.if_range, sub_id) will handle if the sub_id is empty or if the if_range is empty by returning 0 index.

- How to verify it
Verified that no exception is logged upon query in VoQ chassis supervisor.
Other platforms like Linecard, single-asic device - no Change in the SNMP result.

src/sonic_ax_impl/mibs/vendor/cisco/ciscoPfcExtMIB.py

@@ -70,9 +70,6 @@ def get_next(self, sub_id):
         :return: the next sub id.
         """
         try:
-            if not sub_id:
-                return self.if_range[0]
-
             right = bisect_right(self.if_range, sub_id)
             if right >= len(self.if_range):
                 return None

tests/namespace/test_pfc.py

@@ -2,6 +2,11 @@
 import sys
 import importlib
 
+if sys.version_info.major == 3:
+    from unittest import mock
+else:
+    import mock
+
 # noinspection PyUnresolvedReferences
 import tests.mock_tables.dbconnector
 
@@ -265,6 +270,18 @@ def test_getPfcSubtree(self):
         self.assertEqual(str(value0.name), str(expected_oid))
         self.assertEqual(value0.data, 209347219842134092490 % pow(2, 64)) # Test integer truncation
 
+    def test_no_interfaces(self):
+        # Test the scenario where there are no interfaces
+        # like the case of Packet-chassis supervisor
+        ciscoPfcExtMIB.cpfcIfTable.pfc_updater.if_range = []
+        importlib.reload(ciscoPfcExtMIB)
+        oid = ObjectIdentifier(32, 0, 0, 0, (1, 3, 6, 1, 4, 1, 9, 9, 813, 1))
+        sub_id = ()
+        with mock.patch('sonic_ax_impl.mibs.logger.error') as mock_logger:
+            get_pdu = ciscoPfcExtMIB.cpfcIfTable.pfc_updater.get_next(sub_id)
+            self.assertEqual(get_pdu, None)
+            mock_logger.assert_not_called()
+
     @classmethod
     def tearDownClass(cls):
         tests.mock_tables.dbconnector.clean_up_config()