feat: Adding validation to Cloudformation to reject same template_body (#4351)

moto/cloudformation/responses.py

@@ -9,7 +9,7 @@ from moto.core.utils import amzn_request_id
 from moto.s3 import s3_backend
 from moto.core import ACCOUNT_ID
 from .models import cloudformation_backends
-from .exceptions import ValidationError
+from .exceptions import ValidationError, MissingParameterError
 from .utils import yaml_tag_constructor
 
 
@@ -64,6 +64,31 @@ class CloudFormationResponse(BaseResponse):
         key = s3_backend.get_object(bucket_name, key_name)
         return key.value.decode("utf-8")
 
+    def _get_params_from_list(self, parameters_list):
+        # Hack dict-comprehension
+        return dict(
+            [
+                (parameter["parameter_key"], parameter["parameter_value"])
+                for parameter in parameters_list
+            ]
+        )
+
+    def _get_param_values(self, parameters_list, existing_params):
+        result = {}
+        for parameter in parameters_list:
+            if parameter.keys() >= {"parameter_key", "parameter_value"}:
+                result[parameter["parameter_key"]] = parameter["parameter_value"]
+            elif (
+                parameter.keys() >= {"parameter_key", "use_previous_value"}
+                and parameter["parameter_key"] in existing_params
+            ):
+                result[parameter["parameter_key"]] = existing_params[
+                    parameter["parameter_key"]
+                ]
+            else:
+                raise MissingParameterError(parameter["parameter_key"])
+        return result
+
     def create_stack(self):
         stack_name = self._get_param("StackName")
         stack_body = self._get_param("TemplateBody")
@@ -81,13 +106,8 @@ class CloudFormationResponse(BaseResponse):
             )
             return 400, {"status": 400}, template.render(name=stack_name)
 
-        # Hack dict-comprehension
-        parameters = dict(
-            [
-                (parameter["parameter_key"], parameter["parameter_value"])
-                for parameter in parameters_list
-            ]
-        )
+        parameters = self._get_params_from_list(parameters_list)
+
         if template_url:
             stack_body = self._get_stack_from_s3_url(template_url)
         stack_notification_arns = self._get_multi_param("NotificationARNs.member")
@@ -312,6 +332,22 @@ class CloudFormationResponse(BaseResponse):
         template = self.response_template(GET_TEMPLATE_SUMMARY_TEMPLATE)
         return template.render(template_summary=template_summary)
 
+    def _validate_different_update(self, incoming_params, stack_body, old_stack):
+        if incoming_params and stack_body:
+            new_params = self._get_param_values(incoming_params, old_stack.parameters)
+            if old_stack.template == stack_body and old_stack.parameters == new_params:
+                raise ValidationError(
+                    old_stack.name, message=f"Stack [{old_stack.name}] already exists",
+                )
+
+    def _validate_status(self, stack):
+        if stack.status == "ROLLBACK_COMPLETE":
+            raise ValidationError(
+                stack.stack_id,
+                message="Stack:{0} is in ROLLBACK_COMPLETE state and can not "
+                "be updated.".format(stack.stack_id),
+            )
+
     def update_stack(self):
         stack_name = self._get_param("StackName")
         role_arn = self._get_param("RoleARN")
@@ -336,13 +372,8 @@ class CloudFormationResponse(BaseResponse):
             tags = None
 
         stack = self.cloudformation_backend.get_stack(stack_name)
-        if stack.status == "ROLLBACK_COMPLETE":
-            raise ValidationError(
-                stack.stack_id,
-                message="Stack:{0} is in ROLLBACK_COMPLETE state and can not be updated.".format(
-                    stack.stack_id
-                ),
-            )
+        self._validate_different_update(incoming_params, stack_body, stack)
+        self._validate_status(stack)
 
         stack = self.cloudformation_backend.update_stack(
             name=stack_name,

tests/test_cloudformation/test_cloudformation_stack_crud_boto3.py

@@ -3,12 +3,10 @@ from __future__ import unicode_literals
 import json
 from collections import OrderedDict
 from datetime import datetime, timedelta
-from freezegun import freeze_time
 import pytz
 
 import boto3
-from botocore.exceptions import ClientError, ValidationError
-import sure  # noqa
+from botocore.exceptions import ClientError
 
 import pytest
 
@@ -1023,6 +1021,43 @@ def test_boto3_update_stack_fail_missing_new_parameter():
         )
 
 
+@mock_cloudformation
+def test_boto3_update_stack_fail_update_same_template_body():
+
+    name = "update_stack_with_previous_value"
+    cf_conn = boto3.client("cloudformation", region_name="us-east-1")
+    params = [
+        {"ParameterKey": "TagName", "ParameterValue": "foo"},
+        {"ParameterKey": "TagDescription", "ParameterValue": "bar"},
+    ]
+
+    cf_conn.create_stack(
+        StackName=name, TemplateBody=dummy_template_yaml_with_ref, Parameters=params,
+    )
+
+    with pytest.raises(ClientError) as exp:
+        cf_conn.update_stack(
+            StackName=name,
+            TemplateBody=dummy_template_yaml_with_ref,
+            Parameters=params,
+        )
+    exp_err = exp.value.response.get("Error")
+    exp_metadata = exp.value.response.get("ResponseMetadata")
+
+    exp_err.get("Code").should.equal("ValidationError")
+    exp_err.get("Message").should.equal(f"Stack [{name}] already exists")
+    exp_metadata.get("HTTPStatusCode").should.equal(400)
+
+    cf_conn.update_stack(
+        StackName=name,
+        TemplateBody=dummy_template_yaml_with_ref,
+        Parameters=[
+            {"ParameterKey": "TagName", "ParameterValue": "new_foo"},
+            {"ParameterKey": "TagDescription", "ParameterValue": "new_bar"},
+        ],
+    )
+
+
 @mock_cloudformation
 def test_boto3_update_stack_deleted_resources_can_reference_deleted_parameters():