JSON parse error inprintStackDiff leading to malformed diffs

[k7k0]

Describe the bug
When running a stack diff, the printStackDiff function is doing a JSON.stringify of newTemplate which is the template path, not the contents. Leading to diffs like this:

Stack: AWSAccelerator-NetworkAssociationsGwlbStack-<account>-<region>
Other Changes
[+] Unknown 270: l
[+] Unknown 271: a
[+] Unknown 272: t
[+] Unknown 273: e
[+] Unknown 274: .
[+] Unknown 275: j
[+] Unknown 276: s
[+] Unknown 277: o
[+] Unknown 278: n
[~] Unknown 266: j to t
[~] Unknown 267: s to e
[~] Unknown 268: o to m
[~] Unknown 269: n to 

instead it should do JSON.stringify(readTemplate(newTemplate)) to stringify the file contents and get the correct mangled version.

To Reproduce
Synth and diff a stage having differenceCount bigger than the number of letters of the .json path length.
For example, add a new region and a VPC, then

yarn ts-node --transpile-only cdk.ts --require-approval never --partition aws --stage network-associations --config-dir <config- dir> synth
yarn ts-node --transpile-only cdk.ts --require-approval never --partition aws --stage network-associations --config-dir <config-dir> diff

Expected behavior

Stack: AWSAccelerator-NetworkAssociationsGwlbStack-<account>-<region>
Template
[+] Description Description: (SO0199-networkgwlb) Landing Zone Accelerator on AWS. Version 1.9.2.

Parameters
[+] Parameter ###
....

Resources
[+] AWS::SSM::Parameter ###
[+] AWS::SSM::Parameter ###

Please complete the following information about the solution:

• Version: 1.9.2
• Region: any
• Was the solution modified from the version published on this repository? No
• If the answer to the previous question was yes, are the changes available on GitHub?
• Have you checked your service quotas for the services this solution uses? N/A
• Were there any errors in the CloudWatch Logs? N/A

[richardkeit]

Smart @k7k0!

Related issue:

• Pipeline Stage Diff gives limited insight into changes #577

[k7k0]

it looks like a regression introduced while upgrading to CDK 2.148.0.

readTemplate should be used for the fullDiff on next line as well

[richardkeit]

@erwaxler - please mark as closed.
Thanks