chassisd: Fix crash on exit on linecard

Set the `config_manager` variable to `None` if we are running on a
linecard and thus don't need to set up the config manager.

During cleanup, the chassid service tries to clean up the
`config_manager`, but the `config_manager` variable is only ever
initialized if we are on the supervisor. Thus, checking if it is
`None` is insufficient because this results in an `UnboundLocalError`
that prevents the cleanup from succeeding on a linecard.

sonic-chassisd/scripts/chassisd

@@ -435,6 +435,8 @@ class ChassisdDaemon(daemon_base.DaemonBase):
         if self.module_updater.supervisor_slot == self.module_updater.my_slot:
             config_manager = ConfigManagerTask()
             config_manager.task_run()
+        else:
+            config_manager = None
 
         # Start main loop
         self.log_info("Start daemon main loop")

sonic-chassisd/tests/mock_swsscommon.py

@@ -30,3 +30,18 @@ class FieldValuePairs:
     def __init__(self, fvs):
         self.fv_dict = dict(fvs)
         pass
+
+class Select:
+    TIMEOUT = 1
+
+    def addSelectable(self, selectable):
+        pass
+
+    def removeSelectable(self, selectable):
+        pass
+
+    def select(self, timeout=-1, interrupt_on_signal=False):
+        return self.TIMEOUT, None
+
+class SubscriberStateTable(Table):
+    pass

sonic-chassisd/tests/test_chassisd.py

@@ -507,9 +507,20 @@ def test_signal_handler():
     assert daemon_chassisd.stop.set.call_count == 0
     assert exit_code == 0
 
-def test_daemon_run():
+def test_daemon_run_supervisor():
     # Test the chassisd run
     daemon_chassisd = ChassisdDaemon(SYSLOG_IDENTIFIER)
     daemon_chassisd.stop = MagicMock()
     daemon_chassisd.stop.wait.return_value = True
     daemon_chassisd.run()
+
+def test_daemon_run_linecard():
+    # Test the chassisd run
+    daemon_chassisd = ChassisdDaemon(SYSLOG_IDENTIFIER)
+    daemon_chassisd.stop = MagicMock()
+    daemon_chassisd.stop.wait.return_value = True
+
+    import sonic_platform.platform
+    with patch.object(sonic_platform.platform.Chassis, 'get_my_slot') as mock:
+       mock.return_value = sonic_platform.platform.Platform().get_chassis().get_supervisor_slot() + 1
+       daemon_chassisd.run()