Add overrides for aws_sns_topic and other problematic conversions

[jignesh1995]

This is an excellent python app for translating Cloudformation Stack to Terraform scripts

Some modules could however use an improvement while translating to terraform scripts:

• aws_autoscaling_notification
• aws_autoscaling_groups
• aws_acmpca_policy
• aws_cloudwatch_composite_alarm
• aws_sns_topic

The generated script has terraform blocks such as the following:

resource "aws_sns_topic" "some_topic" {
  name = "${var.environment_name}-some-topic"
  // CF Property(Subscription) = [
  //   {
  //     Endpoint = aws_lambda_function.lambda_function_name.arn
  //     Protocol = "lambda"
  //   }
  // ]
}

When in ideal scenario, there should have been 2 resources (1 for SNS Topic, 1 for SNS Subscription):

resource "aws_sns_topic" "some_topic" {
  name = "${var.environment_name}-some-topic"
}

resource "aws_sns_topic_subscription" some_sqs_target" {
  topic_arn = aws_sns_topic.some_topic.arn
  protocol  = "lambda"
  endpoint  = "aws_lambda_function.lambda_function_name.arn"
}

I think the primary reason for this is that my Cloudformation script is creating the resources within the same module(CF block Type: "AWS::SNS::Topic"), however terraform based on its API documentation requires multiple modules to replicate the same resource structure.

PS: Props to the idea of not failing the conversion for such scenarios and instead adding a comment block at the appropriate locations//CF Property.... so that developers can resolve them manually.

[shadycuz]

@jignesh1995 That is exactly what is happening. The converter tries to convery the Cloudformation property "Subscription" into a Terraform attribute for aws_sns_topic but it doesn't exist, so it gives up.

This is something could be fixed. I added an override system to the tool in a previous release. This override functionality allows you to inspect the Cloudformation properties ahead of time and then manually do things.

The logic for performing the overrides is here:

cf2tf/src/cf2tf/convert.py

Lines 610 to 625 in a0ad1ea

	def perform_resource_overrides(
	tf_type: str, params: Dict[str, TerraformType], tc: TemplateConverter
	):
	log.debug("Overiding params for {tf_type}")
	if tf_type not in OVERRIDE_DISPATCH:
	return params
	param_overrides = OVERRIDE_DISPATCH[tf_type]
	for param, override in param_overrides.items():
	if param in params:
	params = override(tc, params)
	return params

The actual overrides are stored here:

cf2tf/src/cf2tf/conversion/overrides.py

Lines 1 to 52 in a0ad1ea

	from typing import TYPE_CHECKING, Callable, Dict
	from cf2tf.terraform.hcl2.custom import LiteralType
	from cf2tf.terraform.hcl2.primitive import NullType, StringType, TerraformType
	if TYPE_CHECKING:
	from cf2tf.convert import TemplateConverter
	CFParams = Dict[str, TerraformType]
	Override = Callable[["TemplateConverter", CFParams], CFParams]
	ParamOverride = Dict[str, Override]
	ResourceOverride = Dict[str, ParamOverride]
	def s3_bucket_acl(_tc: "TemplateConverter", params: CFParams) -> CFParams:
	access_controls: CFParams = {
	"Private": StringType("private"),
	"PublicRead": StringType("public-read"),
	"PublicReadWrite": StringType("public-read-write"),
	"AuthenticatedRead": StringType("authenticated-read"),
	"LogDeliveryWrite": StringType("log-delivery-write"),
	"BucketOwnerRead": NullType(),
	"BucketOwnerFullControl": NullType(),
	"AwsExecRead": StringType("aws-exec-read"),
	}
	orig_value = params["AccessControl"]
	new_value = access_controls[orig_value] # type: ignore
	del params["AccessControl"]
	params["acl"] = new_value
	return params
	def s3_bucket_policy(_tc: "TemplateConverter", params: CFParams) -> CFParams:
	policy_value = params["PolicyDocument"]
	params["PolicyDocument"] = LiteralType(f"jsonencode({policy_value.render(4)}\n )")
	return params
	OVERRIDE_DISPATCH: ResourceOverride = {
	"aws_s3_bucket": {"AccessControl": s3_bucket_acl},
	"aws_s3_bucket_policy": {"PolicyDocument": s3_bucket_policy},
	}

I will take a look at fixing the topic subscription with an override soon. Thanks for opening the issue with a clear example.