[GCU] Prohibit removal of PFC_WD POLL_INTERVAL field (#2777)

What I did
-Add new infrastructure to GCU for validation of moves that YANG models are unable to capture
-Prohibit deletion of PFC_WD POLL_INTERVAL field

How I did it
Ensure JSON Diff does not have POLL_INTERVAL deletion
If diff reflects request for POLL_INTERVAL deletion, raise an Exception and do not allow patch application to go through
How to verify it
Attempt to delete POLL_INTERVAL field using GCU

Author: isabelmsft

generic_config_updater/generic_updater.py

@@ -47,6 +47,10 @@ def apply(self, patch):
         # Generate target config
         self.logger.log_notice("Simulating the target full config after applying the patch.")
         target_config = self.patch_wrapper.simulate_patch(patch, old_config)
+        
+        # Validate all JsonPatch operations on specified fields
+        self.logger.log_notice("Validating all JsonPatch operations are permitted on the specified fields")
+        self.config_wrapper.validate_field_operation(old_config, target_config)
 
         # Validate target config does not have empty tables since they do not show up in ConfigDb
         self.logger.log_notice("Validating target config does not have empty tables, " \

generic_config_updater/gu_common.py

@@ -16,6 +16,12 @@
 class GenericConfigUpdaterError(Exception):
     pass
 
+class IllegalPatchOperationError(ValueError):
+    pass
+
+class EmptyTableError(ValueError):
+    pass
+
 class JsonChange:
     """
     A class that describes a partial change to a JSON object.
@@ -135,6 +141,22 @@ def validate_config_db_config(self, config_db_as_json):
 
         return True, None
 
+    def validate_field_operation(self, old_config, target_config):
+        """
+        Some fields in ConfigDB are restricted and may not allow third-party addition, replacement, or removal. 
+        Because YANG only validates state and not transitions, this method helps to JsonPatch operations/transitions for the specified fields. 
+        """
+        patch = jsonpatch.JsonPatch.from_diff(old_config, target_config)
+        
+        # illegal_operations_to_fields_map['remove'] yields a list of fields for which `remove` is an illegal operation 
+        illegal_operations_to_fields_map = {'add':[],
+                                            'replace': [],
+                                            'remove': ['/PFC_WD/GLOBAL/POLL_INTERVAL', '/PFC_WD/GLOBAL']}
+        for operation, field_list in illegal_operations_to_fields_map.items():
+            for field in field_list:
+                if any(op['op'] == operation and field == op['path'] for op in patch):
+                    raise IllegalPatchOperationError("Given patch operation is invalid. Operation: {} is illegal on field: {}".format(operation, field))
+
     def validate_lanes(self, config_db):
         if "PORT" not in config_db:
             return True, None

tests/generic_config_updater/gu_common_test.py

@@ -69,6 +69,18 @@ def setUp(self):
         self.config_wrapper_mock = gu_common.ConfigWrapper()
         self.config_wrapper_mock.get_config_db_as_json=MagicMock(return_value=Files.CONFIG_DB_AS_JSON)
 
+    def test_validate_field_operation_legal(self):
+        old_config = {"PFC_WD": {"GLOBAL": {"POLL_INTERVAL": "60"}}}
+        target_config = {"PFC_WD": {"GLOBAL": {"POLL_INTERVAL": "40"}}}
+        config_wrapper = gu_common.ConfigWrapper()
+        config_wrapper.validate_field_operation(old_config, target_config)
+    
+    def test_validate_field_operation_illegal(self):
+        old_config = {"PFC_WD": {"GLOBAL": {"POLL_INTERVAL": 60}}}
+        target_config = {"PFC_WD": {"GLOBAL": {}}}
+        config_wrapper = gu_common.ConfigWrapper()
+        self.assertRaises(gu_common.IllegalPatchOperationError, config_wrapper.validate_field_operation, old_config, target_config)
+
     def test_ctor__default_values_set(self):
         config_wrapper = gu_common.ConfigWrapper()