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feat(event-handler): throw error when middleware does not await next() #4511

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feat(event-handler): throw error when middleware does not await next() #4511

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19 changes: 16 additions & 3 deletions packages/event-handler/src/rest/utils.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -172,9 +172,22 @@ export const composeMiddleware = (middleware: Middleware[]): Middleware => {
}

const middlewareFn = middleware[i];
const middlewareResult = await middlewareFn(params, reqCtx, () =>
dispatch(i + 1)
);
let nextPromise: Promise<void> | null = null;
let nextAwaited = false;
const nextFn = async () => {
nextPromise = dispatch(i + 1);
const result = await nextPromise;
nextAwaited = true;
return result;
};

const middlewareResult = await middlewareFn(params, reqCtx, nextFn);

if (nextPromise && !nextAwaited && i < middleware.length - 1) {
throw new Error(
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What do you think about logging a warning instead?

As the error message hints at, we don't know for sure if this will cause issues, so there's a chance we're throwing a 500 error and breaking the customer API prematurely.

If we log a warning instead, at best we are helping them solve these subtle bugs, at worst we just let them know and they can remove the warning by fixing it.

Also can we in any way include the middleware name (aka function name) in the message?

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If a middleware can be used to control fine-grained authorization on a route/global, for example, and potentially executes out of order, allowing other middleware to execute even without proper authorization and introducing potential security issues, I believe this should thrown an error for the developer to fix in the DEV ENV. Also, make it super cleat in the doc.
If not and a middleware can only be used for other purposes, a warning makes more sense.

In Python, customers use middleware to execute custom authN and AuthZ.

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@svozza svozza Sep 19, 2025 •
edited
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I'm open to persuasion here but this is my reasoning:

  • The execution order is a fundamental invariant of the composition algorithm, if this does not hold the system is in an invalid state regardless of whether it accidentally works.
  • Throwing means this is far more likely to get caught at the unit test level before it ever gets to production. A warning is very easy to miss especially when you consider test frameworks such as Vitest suppress console output when running tests.
  • Any post processing middleware that interacts with reqCtx.res.body even something as innoucous as reqCtx.res = new Response(reqCtx.res.body) will throw an error. Which would you rather receive as an error in prod: TypeError [ERR_INVALID_STATE]: Invalid state: The ReadableStream is locked or 'Middleware called next() without awaiting. This may lead to unexpected behavior.?

Regarding the middleware name, I had thought that we could add the number in the chain, ie., the middleware at position ${i} in the middleware stack did not call next. This might make it easier to find because a lot of middleware could just be anonymous functions but maybe we could do the function name and fall back to the location if the function is anonymous.

'Middleware called next() without awaiting. This may lead to unexpected behavior.'
);
}

if (middlewareResult !== undefined) {
result = middlewareResult;
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26 changes: 26 additions & 0 deletions packages/event-handler/tests/unit/rest/Router/middleware.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -180,6 +180,32 @@ describe('Class: Router - Middleware', () => {
expect(body.message).toContain('next() called multiple times');
});

it('should throw error if middleware does not await next()', async () => {
// Prepare
vi.stubEnv('POWERTOOLS_DEV', 'true');
const app = new Router();

app.use(async (_params, _reqCtx, next) => {
await next();
});

app.use(async (_params, _reqCtx, next) => {
next();
});

// Act
const result = await app.resolve(
createTestEvent('/test', 'OPTIONS'),
context
);

// Assess
const body = JSON.parse(result.body);
expect(body.message).toEqual(
'Middleware called next() without awaiting. This may lead to unexpected behavior.'
);
});

it('handles errors thrown in middleware before next()', async () => {
// Prepare
const app = new Router();
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