core: Double nested stack outputs not resolved correctly

When multiple stacks are nested inside of each other, accessing resources that depend on parameters doesn’t seem to work correctly. I don’t have a good explanation of what is happening, but I have multiple ways to fix whatever the problem is.

The CloudFormation I’m trying to import that is failing looks like:

root-stack.yaml
       |----------load-balancers.yaml
               |----------load-balancer-stack-1.yaml
               |----------load-balancer-stack-2.yaml
               |----------load-balancer-stack-3.yaml

The problem we’re running into is using the TargetGroup from load-balancer-stack-1.yaml in some new CDK resources.

This is related to https://github.com/aws/aws-cdk/issues/1374

Reproduction Steps

bin/cdk-test.ts:

#!/usr/bin/env node
import { App, Stack } from '@aws-cdk/core';
import { CfnInclude } from '@aws-cdk/cloudformation-include';
import { CfnBucket, Bucket } from '@aws-cdk/aws-s3';

class DoubleNestedStack extends Stack {
  readonly outputBucketName : CfnBucket;

  constructor(scope: App, id: string) {
      super(scope, id);

    const parentTemplate = new CfnInclude(this, 'ParentStack', {
        templateFile: 'parent-stack.yaml',
    });
    const middleTemplate = parentTemplate.loadNestedStack('MiddleStack', {
        templateFile: 'middle-stack.yaml'
    }).includedTemplate;
    const bottomTemplate = middleTemplate.loadNestedStack('BottomStack', {
        templateFile: 'bottom-stack.yaml'
    }).includedTemplate;

    this.outputBucketName = bottomTemplate.getResource('Bucket') as CfnBucket;
  }
}

class OtherStack extends Stack {
    constructor(scope: App, id: string, bucketName: string) {
        super(scope, id);

        const newBucket = new Bucket(this, 'newBucket', {
            bucketName: bucketName
        });
    }
}

const app = new App();
const doubleNestedStack = new DoubleNestedStack(app, 'DoubleNestedStack');
const bucketName = doubleNestedStack.outputBucketName.bucketName as string;
new OtherStack(app, 'OtherStack', bucketName);

parent-stack.yaml:

AWSTemplateFormatVersion: '2010-09-09'

Resources:
  MiddleStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: 'https://s3.amazonaws.com/this-doesnt-matter.yaml'

middle-stack.yaml:

AWSTemplateFormatVersion: '2010-09-09'

Resources:
  BottomStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: 'https://s3.amazonaws.com/this-doesnt-matter.yaml'
      Parameters:
        Name: anything

bottom-stack.yaml:

AWSTemplateFormatVersion: '2010-09-09'

Parameters:
  Name:
    Type: String

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Ref Name

What did you expect to happen?

I expected cdk synth to work and my OtherStack to be able to reference the double nested stack’s resource. (I realize that I’m trying to name an S3 bucket the same thing as another bucket, but that’s not the problem)

[masonme@f45c89b36d13] $ cdk synth
Successfully synthesized to /Users/masonme/cdk-migration-test/cdk.out
Supply a stack id (DoubleNestedStack, OtherStack) to display its template.

What actually happened?

Failure Output:

[masonme@f45c89b36d13] $ cdk synth

/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/construct-compat.ts:83

            ^
Error: Cannot use tokens in construct ID: DoubleNestedStackParentStackMiddleStackBottomStackNestedStackBottomStackNestedStackResource43C48F46Outputs.${Token[DoubleNestedStack.ParentStack.MiddleStack.Middl...arentStackMiddleStackBottomStackName8BE86CBFRef.LogicalID.48]}
    at new Construct (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/construct-compat.ts:83:13)
    at new CfnElement (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/cfn-element.ts:57:5)
    at new CfnOutput (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/cfn-output.ts:50:5)
    at createNestedStackOutput (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/private/refs.ts:207:14)
    at resolveValue (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/private/refs.ts:86:25)
    at resolveValue (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/private/refs.ts:87:12)
    at Object.resolveReferences (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/private/refs.ts:30:24)
    at Object.prepareApp (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/private/prepare-app.ts:31:3)
    at Object.synthesize (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/private/synthesis.ts:24:3)
    at App.synth (/Users/masonme/cdk-migration-test/node_modules/@aws-cdk/core/lib/stage.ts:180:23)
Subprocess exited with error 1

cdk synth failed due to CDK using a token in the ID of an internal construct. The code for this ID seems to be here: https://github.com/aws/aws-cdk/blob/7966f8d48c4bff26beb22856d289f9d0c7e7081d/packages/%40aws-cdk/core/lib/private/refs.ts#L204

I’ve found multiple ways to avoid the problem:

• Change bottom-stack.yaml’s BucketName property to a string instead of a reference to a parameter
• RemoveOtherStack or its usage of BottomStack’s output

Environment

• CDK CLI Version : 1.103.0 (build bc13a66)
• Framework Version: 1.103.0
• Node.js Version: v14.15.4
• OS : OS X 10.14.6 (18G9216)
• Language (Version): TypeScript (Version 4.1.5)

Other

---

This is 🐛 Bug Report

[eladb]

I’ll take a look next week

[github-actions[bot]]

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