Contextvars propagation to back ground task

[yyamano]

Is it possible to propapate contextvars to a back ground task?

Here is my simple server code.

import contextvars
from time import sleep
import uuid

from starlette.applications import Starlette
from starlette.responses import JSONResponse
from starlette.routing import Route
from starlette.background import BackgroundTasks

request_id = contextvars.ContextVar('request_id')
def bar():
    print(f"bar: request_id={request_id.get()}")
    
def foo(request):
    request_id.set(uuid.uuid4())
    print(f"foo: request_id={request_id.get()}")
    tasks = BackgroundTasks()
    tasks.add_task(bar)
    return JSONResponse({'message': 'foo'}, background=tasks)
    
routes = [
    Route('/foo', endpoint=foo, methods=['GET'])
]
app = Starlette(routes=routes)

I get the following error when I send a GET request to http://localhost:8000/foo.

> poetry run uvicorn buz:app
INFO:     Started server process [22440]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
foo: request_id=c6b3dac2-62f4-4525-a1d3-d9f6bd5c3d69
INFO:     127.0.0.1:49240 - "GET /foo HTTP/1.1" 200 O

[snip]

  File "C:\Users\yyamano\work\buz.py", line 12, in bar
    print(f"bar: request_id={request_id.get()}")
LookupError: <ContextVar name='request_id' at 0x000002238E9AACF0>

[alex-oleshkevich]

1. Try to make foo async function.
2. In this example, you'd better to pass request id directly to the task without using globals.
   I personally try to avoid any globals if possible