Do not send response to CloudFormation when testing with sam local

[ycourtois]

When testing my custom resource with sam local, the resource helper tries to send a response to CloudFormation.

It generates an error since I am using fake information in my request payload.

Example:

{
  "RequestType": "Create",
  "ResponseURL": "http://pre-signed-S3-url-for-response",
  "StackId": "arn:aws:cloudformation:us-east-1:123456789012:stack/MyStack/guid",
  "RequestId": "unique id for this create request",
  "ResourceType": "Custom::TestResource",
  "LogicalResourceId": "MyTestResource",
...

It would be nice to disable response sending to cfn when testing with sam local.

crhelper version: 2.0.10

[iainelder]

You can do this by mocking the _send method of the CfnResource class.

The normal implementation builds a response body. This behavior you want to keep so you can test the response.

_send passes the response body to a function parameter called send_response which normally writes the response body to the ResponseURL. This writing behavior you want to remove.

So if you replace _send with a wrapped version that sets send_response to a Mock instance, and you keep a a handle on the mock in your test, you should be able to get the response body from the called_args of the mock and assert its contents.

custom-resource-helper/crhelper/resource_helper.py

Lines 225 to 242 in 3c7e40e

	def _send(self, status=None, reason="", send_response=_send_response):
	if len(str(str(self.Reason))) > 256:
	self.Reason = "ERROR: (truncated) " + str(self.Reason)[len(str(self.Reason)) - 240:]
	if len(str(reason)) > 256:
	reason = "ERROR: (truncated) " + str(reason)[len(str(reason)) - 240:]
	response_body = {
	'Status': self.Status,
	'PhysicalResourceId': str(self.PhysicalResourceId),
	'StackId': self.StackId,
	'RequestId': self.RequestId,
	'LogicalResourceId': self.LogicalResourceId,
	'Reason': str(self.Reason),
	'Data': self.Data,
	'NoEcho': self.NoEcho,
	}
	if status:
	response_body.update({'Status': status, 'Reason': reason})
	send_response(self._response_url, response_body, self._ssl_verify)

I'd love to see a built-in way of supporting such testing without so much monkey patching :-)

[verbitan]

@iainelder please forgive my ignorance, but are you able to explain why this doesn't work?

    @patch('crhelper.utils._send_response')
    @patch('crhelper.resource_helper.CfnResource._set_timeout', Mock())
    def test_create(self, mock_send):
        event = {
            "RequestType": "Create",
            "RequestId": "test-event-id",
            "StackId": "arn/test-stack-id/guid",
            "LogicalResourceId": "TestResourceId",
            "ResponseURL": "response_url",
            "ResourceProperties": {
                "MyData": "SomeProperty"
            }
        }

        lambda_function.handler(event, "")

        # Do some asserting on mock_send.call_args_list

[iainelder]

@verbitan , it's not a complete example and you haven't shown me what you expect or what you get instead. So it's hard to say why it doesn't work.

It's been a while since I've used crhelper, but I did develop a working solution for this on a previous project.

I've extracted from it a minimal example and I've published it to a new GitHub repo: https://github.com/iainelder/custom-resource-helper-mock

You should be able to set up the environment and run the example in PyTest. The mocked_send_response function is the core of the solution.

Does that help? Can you adapt it for your use case?

[verbitan]

That's perfect @iainelder, thank you!