[system-health] When disabling a feature the SYSTEM_READY|SYSTEM_STATE was not updated (#14823)

- Why I did it
If you enable feature and then disable it, System Ready status change to Not Ready

A disabled feature should not affect the system ready status.

- How I did it
During the disable flow of dhcp_relay, it entered the dnsrvs_name list, which caused the SYSTEM_STATE key to be set to DOWN. Right after that, the dhcp_relay service was removed from the full service list, however, but, when it was removed from the dnsrvs_name, there was no flow to reset the system state back to UP even though there was no more services in down state.

- How to verify it
root@qa-eth-vt01-2-3700v:/home/admin# config feature state dhcp_relay enabled 
root@qa-eth-vt01-2-3700v:/home/admin# show system-health sysready-status 

root@qa-eth-vt01-2-3700v:/home/admin# config feature state dhcp_relay disabled
root@qa-eth-vt01-2-3700v:/home/admin# show system-health sysready-status 

Should see
System is ready

src/system-health/health_checker/sysmonitor.py

@@ -406,6 +406,12 @@ class Sysmonitor(ProcessTaskBase):
             if event in self.dnsrvs_name:
                 self.dnsrvs_name.remove(event)
 
+            if len(self.dnsrvs_name) == 0:
+                astate = "UP"
+            else:
+                astate = "DOWN"
+            self.publish_system_status(astate)
+
             srv_name,last = event.split('.')
             # stop on service maybe propagated to timers and in that case,
             # the state_db entry for the service should not be deleted

src/system-health/tests/mock_connector.py

@@ -23,6 +23,9 @@ class MockConnector(object):
     def get_all(self, db_id, key):
         return MockConnector.data[key]
 
+    def exists(self, db_id, key):
+        return key in MockConnector.data
+
     def set(self, db_id, key, field, value):
         self.data[key] = {}
         self.data[key][field] = value

src/system-health/tests/test_system_health.py

@@ -667,6 +667,31 @@ def test_check_unit_status():
     assert 'mock_bgp.service' in sysmon.dnsrvs_name
 
 
+@patch('health_checker.sysmonitor.Sysmonitor.get_all_service_list', MagicMock(side_effect=[
+    ['mock_snmp.service', 'mock_bgp.service', 'mock_ns.service'],
+    ['mock_snmp.service', 'mock_ns.service']
+]))
+@patch('health_checker.sysmonitor.Sysmonitor.run_systemctl_show', MagicMock(return_value=mock_srv_props['mock_bgp.service']))
+@patch('health_checker.sysmonitor.Sysmonitor.get_app_ready_status', MagicMock(return_value=('Down','-','-')))
+@patch('health_checker.sysmonitor.Sysmonitor.post_unit_status', MagicMock())
+@patch('health_checker.sysmonitor.Sysmonitor.print_console_message', MagicMock())
+def test_system_status_up_after_service_removed():
+    sysmon = Sysmonitor()
+    sysmon.publish_system_status('UP')
+
+    sysmon.check_unit_status('mock_bgp.service')
+    assert 'mock_bgp.service' in sysmon.dnsrvs_name
+    result = swsscommon.SonicV2Connector.get(MockConnector, 0, "SYSTEM_READY|SYSTEM_STATE", 'Status')
+    print("system status result before service was removed from system: {}".format(result))
+    assert result == "DOWN"
+
+    sysmon.check_unit_status('mock_bgp.service')
+    assert 'mock_bgp.service' not in sysmon.dnsrvs_name
+    result = swsscommon.SonicV2Connector.get(MockConnector, 0, "SYSTEM_READY|SYSTEM_STATE", 'Status')
+    print("system status result after service was removed from system: {}".format(result))
+    assert result == "UP"
+
+
 @patch('health_checker.sysmonitor.Sysmonitor.get_all_service_list', MagicMock(return_value=['mock_snmp.service']))
 def test_check_unit_status_timer():
     sysmon = Sysmonitor()