Middy: Error handlers not catching properly with async middlewares

[coreylight]

An async middleware with async keyword or just a middleware that implements asynchronous work, doesn't seem to properly bubble up the error object for middlewares that implement the onError method.

This doesn't seem to be properly caught:

export default function() {
  return {
    before: async (handler, next) => {
      throw new Error('wow');
    }
  };
}

Or even something like this:

export default function() {
  return {
    before: (handler, next) => {
      setTimeout(() => {
        throw new Error('wow');
      }, 10);
    }
  };
}

Any ideas?

[coreylight]

Passing the error to next seems to work, but it might be cool if these were caught without that. 🙂

[lmammino]

With the current implementation, middlewares do not support promises as return object (which is essentially what happens behind the scenes of async functions).

I think there's an opportunity here to add support for this to allow middy to be even more expressive and simple to use.

I will investigate this option and add few test cases similar to the ones you mentioned here. Feel free to contribute to some proposal if you want.

At a first glance, my current expectation is that we should handle the possibility that a middleware returns a function here. If this happens, we should use attach to the returned promise a .then(runNext) and handler the possible rejection with .catch((err) => done(err)) (or .catch(done) in brief).

But being late at night, that's just me speculating 😛

Thanks for the meaningful reporting though!

@dkatavic what do you think here?

[dkatavic]

@lmammino I like what you have proposed in PR #51 , async middlewares should be supported out of the box 👍