Bug: Idempotency middleware doesn't work for functions without return value

[wassil]

Expected Behavior

When using the makeHandlerIdempotent middleware with a handler function that has no return value(undefined), the middleware should only call the handler once.

Current Behavior

The handler is called multiple times for the same event, even though the idempotency record exists in Dynamo.

The issue is that the package returns the stored return value from previous run from the middleware.
Here: https://github.com/aws-powertools/powertools-lambda-typescript/blob/main/packages/idempotency/src/IdempotencyHandler.ts#L243
And here: https://github.com/aws-powertools/powertools-lambda-typescript/blob/main/packages/idempotency/src/middleware/makeHandlerIdempotent.ts#L140

If the value returned from middleware is truthy, it causes middy to perform an early return(https://middy.js.org/docs/intro/early-interrupt/).
This works for handler that have a return value, but doesn't work when the return value is undefined - middy doesn't early return and the handler code is executed again.

Code snippet

export handler = middy().use(makeHandlerIdempotent(...)).handler(() => console.log("stuff"))

Steps to Reproduce

Use the code snippet, call the lambda multiple times with the same event

Possible Solution

In order to early return, a truthy value has to be returned from the middleware.
This kind of sucks, because the first time a lambda runs, it returns undefined and we'd have to return something else when idempotency key is present.
In our use case, the return value doesn't matter, but I'm not sure what would make sense as a generic solution.

Powertools for AWS Lambda (TypeScript) version

latest

AWS Lambda function runtime

20.x

Packaging format used

npm

Execution logs

No response

[wassil]

@dreamorosi added support for functions with no return value in #2521

Hope this helps with the triage!

[dreamorosi]

Hi @wassil, thank you for opening the issue and reporting the bug.

I was able to reproduce the behavior you describe and confirm that when the handler function doesn't return a value, the Middy.js middleware runs twice.

We'll look into fixing the issue.

[dreamorosi]

I have spent some time looking at the implementation (thanks for the exhaustive report!) and unfortunately I think we can't do anything unless Middy.js itself supports this use case.

I have opened an issue on their repo (middyjs/middy#1236) and will be waiting to hear from the maintainer(s) on whether this is something that can be supported or not.

I'll wait for a couple of days, after which I'll add a banner to our docs to explain the limitation and suggest customers to use other Idempotency methods (i.e. function wrapper or class method decorator) if their use case can return undefined.