(diff): Support Change Set Diffs for Nested Stacks

[comcalvi]

Describe the feature

#28336 causes diff to create and analyze a change set to more accurately determine resource replacements. This does not support nested stacks, because #18207 inlined the nested templates into the root stack template under a special property, NestedTemplate. When we send this modified root stack template through CreateChangeSet, CFN throws Template Error: Fn::Equals cannot be partially collapsed, because the nested template includes the CDK Metadata condition (which uses Fn::Equals) in the Resources section.

Use Case

using cdk diff with nested stacks.

Proposed Solution

In the toolkit, realize that the template has nested stacks and that we want to create a change set. If so, do not call readCurrentTemplateWithNestedStacks(stack), since this mutates the stack to include the nested templates. Instead, call readCurrentTemplate(stack), which mutates the stack to only include the root stack template. Then, upload the body parameters to S3 for each nested stack (so CFN can find the updated templates for CreateChangeSet) and use each nested change set from DescribeChangeSet() in a similar way that #28336 does to determine resource replacement across the nested stacks.

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

CDK version used

latest

Environment details (OS name and version, etc.)

all