CF - detect changesets without changes (#5058)

moto/cloudformation/models.py

@@ -251,6 +251,17 @@ class FakeStack(BaseModel):
         self.creation_time = datetime.utcnow()
         self.status = "CREATE_PENDING"
 
+    def has_template(self, other_template):
+        our_template = (
+            self.template
+            if isinstance(self.template, dict)
+            else json.loads(self.template)
+        )
+        return our_template == json.loads(other_template)
+
+    def has_parameters(self, other_parameters):
+        return self.parameters == other_parameters
+
     def _create_resource_map(self):
         resource_map = ResourceMap(
             self.stack_id,
@@ -732,6 +743,16 @@ class CloudFormationBackend(BaseBackend):
             tags=tags,
             role_arn=role_arn,
         )
+        if (
+            change_set_type == "UPDATE"
+            and stack.has_template(template)
+            and stack.has_parameters(parameters)
+        ):
+            # Nothing has changed - mark it as such
+            new_change_set.status = "FAILED"
+            new_change_set.execution_status = "UNAVAILABLE"
+            new_change_set.status_reason = "The submitted information didn't contain changes. Submit different information to create a change set."
+        else:
             new_change_set.status = "CREATE_COMPLETE"
             new_change_set.execution_status = "AVAILABLE"
         self.change_sets[change_set_id] = new_change_set

tests/test_cloudformation/test_cloudformation_stack_crud_boto3.py

@@ -1641,6 +1641,83 @@ def test_delete_change_set():
     cf_conn.list_change_sets(StackName="NewStack")["Summaries"].should.have.length_of(0)
 
 
+@mock_cloudformation
+@mock_ec2
+def test_create_change_set_twice__no_changes():
+    cf_client = boto3.client("cloudformation", region_name="us-east-1")
+
+    # Execute once
+    change_set_id = cf_client.create_change_set(
+        StackName="NewStack",
+        TemplateBody=dummy_template_json,
+        ChangeSetName="NewChangeSet",
+        ChangeSetType="CREATE",
+    )["Id"]
+    cf_client.execute_change_set(ChangeSetName=change_set_id, DisableRollback=False)
+
+    # Execute twice
+    change_set_id = cf_client.create_change_set(
+        StackName="NewStack",
+        TemplateBody=dummy_template_json,
+        ChangeSetName="NewChangeSet",
+        ChangeSetType="UPDATE",
+    )["Id"]
+    execution = cf_client.describe_change_set(ChangeSetName=change_set_id)
+
+    # Assert
+    execution["ExecutionStatus"].should.equal("UNAVAILABLE")
+    execution["Status"].should.equal("FAILED")
+    execution["StatusReason"].should.equal(
+        "The submitted information didn't contain changes. Submit different information to create a change set."
+    )
+
+
+@mock_cloudformation
+@mock_ec2
+@mock_s3
+def test_create_change_set_twice__using_s3__no_changes():
+    cf_client = boto3.client("cloudformation", region_name="us-east-1")
+    s3 = boto3.client("s3", region_name="us-east-1")
+    s3_conn = boto3.resource("s3", region_name="us-east-1")
+    s3_conn.create_bucket(Bucket="foobar")
+
+    s3_conn.Object("foobar", "template-key").put(Body=dummy_template_json)
+    key_url_1 = s3.generate_presigned_url(
+        ClientMethod="get_object", Params={"Bucket": "foobar", "Key": "template-key"}
+    )
+
+    s3_conn.Object("foobar", "template-key-unchanged").put(Body=dummy_template_json)
+    key_url_2 = s3.generate_presigned_url(
+        ClientMethod="get_object",
+        Params={"Bucket": "foobar", "Key": "template-key-unchanged"},
+    )
+
+    # Execute once
+    change_set_id = cf_client.create_change_set(
+        StackName="NewStack",
+        TemplateURL=key_url_1,
+        ChangeSetName="NewChangeSet",
+        ChangeSetType="CREATE",
+    )["Id"]
+    cf_client.execute_change_set(ChangeSetName=change_set_id, DisableRollback=False)
+
+    # Execute twice
+    change_set_id = cf_client.create_change_set(
+        StackName="NewStack",
+        TemplateURL=key_url_2,
+        ChangeSetName="NewChangeSet",
+        ChangeSetType="UPDATE",
+    )["Id"]
+    execution = cf_client.describe_change_set(ChangeSetName=change_set_id)
+
+    # Assert
+    execution["ExecutionStatus"].should.equal("UNAVAILABLE")
+    execution["Status"].should.equal("FAILED")
+    execution["StatusReason"].should.equal(
+        "The submitted information didn't contain changes. Submit different information to create a change set."
+    )
+
+
 @mock_cloudformation
 @mock_ec2
 def test_delete_stack_by_name():