Request body empty in FastAPI app that mounts Flask app

[toxygene]

Using Serverless, I'm deploying a FastAPI app to Lambda. The FastAPI app simply mounts a Flask app with a single POST route. When I try to access the request data, it's empty (b'') instead of what was submitted.

from fastapi import FastAPI
from flask import Flask, request
from mangum import Mangum
from starlette.middleware.wsgi import WSGIMiddleware

flask_app = Flask(__name__)


@flask_app.route("/", methods=["POST"])
def hello_world():
    print(request.data)
    return "Hello, World!"


fastapi_app = FastAPI(debug=True)
fastapi_app.mount("/", WSGIMiddleware(flask_app))

handler = Mangum(fastapi_app)

service: fastapi-flask-test
frameworkVersion: '2'

provider:
  name: aws
  runtime: python3.8
  lambdaHashingVersion: 20201221

functions:
  hello:
    handler: handler.handler
    events:
      - http: ANY /
      - http: ANY {proxy+}

plugins:
  - serverless-python-requirements

custom:
  pythonRequirements:
    dockerizePip: true
    dockerSsh: true
    usePoetry: true

If I run this code via uvicorn, the request body is correct.

import uvicorn

from handler import fastapi_app

uvicorn.run(fastapi_app)

[toxygene]

After further investigation, this appears to be a duplicate of #143.