Use asgi root path to handle mount point

[amol-]

Changes chainlit server to rely on ASGI root_path variable to know where the application is mounted on.
This addresses both cases where chainlit is proxied to non root paths, mounted as a subapp or both combined.

I tested this by serving the following app

from fastapi import FastAPI
from chainlit.utils import mount_chainlit

app = FastAPI()

@app.get("/app")
def read_main():
    return {"message": "Hello World from main app"}

mount_chainlit(app=app, target="clapp.py", path="/chainlit")

served with

uvicorn app:app --host 0.0.0.0 --port 80 --root-path=/proxy

The application is correctly available at http://localhost/proxy/chainlit/ and loads, previously it would not load trying to fetch css and js files from wrong path.

This might also fix #1440 #1707 and #1397 but I didn't have the opportunity to confirm that.

[dokterbob]

Great contrib @amol-! Code looks nice and clean, I've been yearning to clean this up myself for some time now!

I'd like to take a bit of time next week to do manual validation and would love feedback from other users, also on #1440 #1707 and #1397.

@duarteocarmo @alexagr @illegalplumbing @marsanla Does this solve your issue?

[dokterbob]

Review progress

• Create test setup with reverse proxy
• Confirm issue with prefix-based reverse proxy on latest chainit
• Confirm resolution on PR
• Confirm functioning with subpath-mounted app on PR
• Confirm resolution of BUG: Double root_path #1397
• Confirm resolution of Chainlit mounting mechanism fails when FastAPI has root_path.  #1440
• Confirm resolution of mount_chainlit() is failing for path="/" #1707
• Publish RP demo app: https://github.com/dokterbob/chainlit-reverse-proxy-demo
• Share RP test app in cookbook: Prefixed reverse proxy cookbook#171

References

• https://fastapi.tiangolo.com/advanced/behind-a-proxy/#providing-the-root_path

[dokterbob]

Looks GREAT to me! Much needed cleanup of unnecessary complexity!