Empty reply data in Fastify async route

[vitalikprac]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.2.0

Plugin version

6.1.0

Node.js version

16.15.0

Operating system

Windows 11

Browser

Chrome 102

Description

The route of fastify has handler function and it's work as expected.

fastify.get('/', (request, reply) => {
    reply.send('hello world');
})

Browser show the correct response headers and render 'hello world' to html

But if we changed handler function to async, it has strange behaviour

fastify.get('/', async(request, reply) => {
    reply.send('hello world');
})

In browser, response headers displays content-length: 0 And browser says that request has no response data available

If I understand correctly https://github.com/fastify/fastify-compress/issues/140 issue should resolve my problem , but it would not work for me.

Steps to Reproduce

1. Init project with yarn or npm
2. Add fastify and @fastify/compress

   {
   "name": "test-fastify",
   "version": "1.0.0",
   "main": "node server.js",
   "type": "module",
   "license": "MIT",
   "dependencies": {
   "@fastify/compress": "^6.1.0",
   "fastify": "^4.2.0"
   }
   }

3. $ yarn install

4. Create server.js file

   
   import Fastify from 'fastify';
   import FastifyCompress from '@fastify/compress';

const fastify = Fastify(); await fastify.register(FastifyCompress, { threshold: 0 }); fastify.get('/', async(request, reply) => { reply.send('hello world'); }) await fastify.listen({ port: 3000 })

5. 
```bash 
$ yarn start

6. Open Browser or Postman at http://localhost:3000
7. View Response directly on the page or in Dev Tools

Expected Behavior

Browser should render hello world in async fastify handler as it does in non-async handler

[mcollina]

Thanks for reporting.

We changed the default behavior in V4, promoting a good promise hygiene.

Your promise returns undefined, so that's what you get.

fastify.get('/', async(request, reply) => {
  return 'hello world'
})

or

fastify.get('/', async(request, reply) => {
  reply.send('hello world')
  return reply
})

[vitalikprac]

Thanks, it's work for me. Also would be nice if you add example with async function to README for better developers experience.