When deployed to vercel sveltekit endpoints have body set to undefined

[ivoreis]

Describe the bug
Running an app and hosting in Vercel (using vercel-adapter) when data is posted to an endpoint containing non-empty body, the req object contains body set to undefined.

Can't reproduce this locally.

• Running npm run dev works as expected.
• Running npm run build && npm run start works as expected.
• Running vercel dev works as expected.

One thing I've noticed is that the req payload in this @svelkit/kit version should already return both body and rawBody but somehow the code deployed to Vercel doesn't return rawBody.

Running latest adapter-vercel:

    "@sveltejs/adapter-vercel": {
      "version": "1.0.0-next.13",
      "resolved": "https://registry.npmjs.org/@sveltejs/adapter-vercel/-/adapter-vercel-1.0.0-next.13.tgz",
      "integrity": "sha512-7/DvjGBW+ldK80E7TsIB3Vpo6QmhdKK1+6pnN3/HrB1Uc+12sxvI76sXQGV0qhcJR3wUdXlHed3p8A8TSx1RhA==",
      "dev": true,
      "requires": {
        "@sveltejs/kit": "1.0.0-next.81",
        "esbuild": "^0.11.12"
      },
      "dependencies": {
        "@sveltejs/kit": {
          "version": "1.0.0-next.81",
          "resolved": "https://registry.npmjs.org/@sveltejs/kit/-/kit-1.0.0-next.81.tgz",
          "integrity": "sha512-v1CxkLYX3xXRdihvpiOOuQs2MZa/f3ENZKeX7sMYDKQ0MzRpmrhzmplVukYD+fZHYUFjMj+yKYPMhOpC/p3ckQ==",
          "dev": true,
          "requires": {
            "@sveltejs/vite-plugin-svelte": "^1.0.0-next.7",
            "cheap-watch": "^1.0.3",
            "sade": "^1.7.4"
          }
        },
        "esbuild": {
          "version": "0.11.12",
          "resolved": "https://registry.npmjs.org/esbuild/-/esbuild-0.11.12.tgz",
          "integrity": "sha512-c8cso/1RwVj+fbDvLtUgSG4ZJQ0y9Zdrl6Ot/GAjyy4pdMCHaFnDMts5gqFnWRPLajWtEnI+3hlET4R9fVoZng==",
          "dev": true
        }
      }
    },

Logs

[POST] /api/reply.json
09:22:19:85
{
  method: 'POST',
  path: '/api/reply.json',
  query: URLSearchParams {},
  body: undefined,
  host: 'svelte-vercel-body.vercel.app',
  params: {},
  context: {}
}

To Reproduce

Request:

POST https://svelte-vercel-body.vercel.app/api/reply.json HTTP/1.1
Content-Type: application/json

{
  "message": "empty body"
}


Response:

{
  "message": {
    "method": "POST",
    "path": "/api/reply.json",
    "query": {},
    "host": "svelte-vercel-body.vercel.app",
    "params": {},
    "context": {}
  }
}

Sample project - https://github.com/ivoreis/svelte-vercel-body

Information about your SvelteKit Installation:

Diagnostics

$ npx envinfo --system --npmPackages svelte,@sveltejs/kit,vite --binaries --browser
npx: installed 1 in 1.186s

System:
OS: macOS 11.2.3
CPU: (16) x64 Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
Memory: 292.50 MB / 32.00 GB
Shell: 5.8 - /bin/zsh
Binaries:
Node: 14.15.3 - /usr/local/bin/node
Yarn: 1.22.4 - ~/.yarn/bin/yarn
npm: 6.14.9 - /usr/local/bin/npm
npmPackages:
@sveltejs/kit: next => 1.0.0-next.84
svelte: ^3.29.0 => 3.37.0
vite: ^2.1.0 => 2.2.1

Browser: Chrome, Safari, Edge, CURL

Severity
How severe an issue is this bug to you? Is this annoying, blocking some users, blocking an upgrade or blocking your usage of SvelteKit entirely?

Note: the more honest and specific you are here the more we will take you seriously.

Non-blocking and I'm aware this tool is a WIP 😊

Please let me know if there's any other detail need and/or anything I can help with.

Additional context
Add any other context about the problem here.

@Rich-Harris Do you think body (https://github.com/sveltejs/kit/blob/master/packages/adapter-vercel/files/entry.js#L15) should be rawBody instead?

Following the code I see https://github.com/sveltejs/kit/blob/master/packages/kit/src/runtime/server/parse_body/index.js#L5-L6 being used by SSR (https://github.com/sveltejs/kit/blob/master/packages/kit/src/runtime/server/index.js#L24) but I'm not familiar with the code to say for certain.