Returning a promise from `requestListener` so we know when processing is done

Description

Hooking bolt into next.js and got it working with:

import { App, HTTPReceiver, GenericMessageEvent } from '@slack/bolt'
import { NextApiRequest, NextApiResponse } from 'next';

const receiver = new HTTPReceiver({
  endpoints: ['/api/slack/events'],
  signingSecret: process.env.SIGNING_SECRET,
  processBeforeResponse: true
})

// Initializes your app with your bot token and the AWS Lambda ready receiver
const app = new App({
  token: process.env.ACCESS_TOKEN,
  receiver,
  ignoreSelf: true
});

app.message('hi', async ({ message, say }) => {
  const msg = message as GenericMessageEvent
  await say(`:wave: <@${msg.user}>`)
})

export default receiver.requestListener;

export const config = {
  api: {
    bodyParser: false,
  }
}

Next.js will complain that:

API resolved without sending a response for /api/slack/events, this may result in stalled requests.

From here: https://github.com/vercel/next.js/issues/10439#issuecomment-583214126 we need to return a promise so it knows when to kill the function on it’s end.

Instead of wrapping in an async closure, can we just return a promise?

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I’ve read and understood the Contributing guidelines and have done my best effort to follow them.
• I’ve read and agree to the Code of Conduct.
• I’ve searched for any related issues and avoided creating a duplicate issue.

[aoberoi]

I got a Next.js app working without the warning. Got it deployed to Vercel as well.

I split things up a bit so that the pages/api/slack/[...all].js file just sets up the HTTP receiver.

Then in lib/bolt.js i did all the normal Bolt things.

What do you think?

[github-actions[bot]]

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