(cli): unable to hotswap when adding tags to lambda function

Describe the bug

The CDK crashes when attempting to add tags to a lambda function while using hotswap.

Expected Behavior

The tags are added to the lambda function.

Current Behavior

 ❌  TestStack failed: TypeError: Cannot read properties of undefined (reading 'forEach')
    at isLambdaFunctionCodeOnlyChange (/Users/test/node_modules/aws-cdk/lib/api/hotswap/lambda-functions.ts:169:32)
    at isHotswappableLambdaFunctionChange (/Users/test/node_modules/aws-cdk/lib/api/hotswap/lambda-functions.ts:34:34)
    at findAllHotswappableChanges (/Users/test/node_modules/aws-cdk/lib/api/hotswap-deployments.ts:96:9)
    at tryHotswapDeployment (/Users/test/node_modules/aws-cdk/lib/api/hotswap-deployments.ts:50:37)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at deployStack (/Users/test/node_modules/aws-cdk/lib/api/deploy-stack.ts:274:39)
    at CdkToolkit.deploy (/Users/test/node_modules/aws-cdk/lib/cdk-toolkit.ts:210:24)
    at initCommandLine (/Users/test/node_modules/aws-cdk/lib/cli.ts:346:12)

Cannot read properties of undefined (reading 'forEach')

Reproduction Steps

• Create lambda function, deploy stack

const func = new lambda.Function(this, "Function", {
  code: lambda.Code.fromInline(`exports.handler = async () => {}`),
  handler: "index.handler",
  runtime: lambda.Runtime.NODEJS_16_X,
});

• Add tags, deploy stack

Tags.of(func).add("key", "value");

cdk deploy --hotswap

Possible Solution

No response

Additional Information/Context

No response

CDK CLI Version

2.37.1

Framework Version

No response

Node.js Version

16.16.0

OS

Mac OS

Language

Typescript

Language Version

3.9.7

Other information

No response

[comcalvi]

yep, this is bad. It turns out that hotswapping tags is entirely broken, and they can't be updated in any way today through hotswap (creating / destroying the Tags prop completely causes this crash, and doing additions / deletions of tags that already have a Tags prop causes a full deployment).

Question: Why do you need to hotswap tags? Why is this something that needs to be done each iteration cycle?

In our team we hotswap by default whenever deploying locally; if we have made changes that can't be hotswapped then we bank on the fact that a full deployment will be done even though the --hotswap argument was included.

If tags cannot be hotswapped, then could it fall back to a full deployment?

[comcalvi]

falling back to a full deployment is the intended behavior. There seems to be no use case for the tags themselves to need to be hotswapped, so we're considering removing the Tag hotswap ability (it doesn't work anyway) and ignoring non-hotswappable property updates in a new flag.