BackgroundTasks can't work with middleware

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Example Code

import asyncio
from fastapi import Request, BackgroundTasks, FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

origins = ["*"]

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)


async def test_sleep():
    print('test')
    await asyncio.sleep(9999)


@app.post("/test")
async def test(background_tasks: BackgroundTasks):
    background_tasks.add_task(test_sleep)
    return 'test'


@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    response = await call_next(request)
    return response

Description

When use middleware, the second request to ‘http://127.0.0.1:8000/test’ will be pending. If I remove middleware, everything works fine. I know it’s the middleware problem, but I don’t known how to fix it.

Operating System

Linux

Operating System Details

Ubuntu 20.04

FastAPI Version

0.66.1

Python Version

3.9.5

Additional Context

No response

[havardthom]

This is a known starlette issue https://github.com/encode/starlette/issues/919

[yujiri8]

I’m seeing a probably related issue but with different symptoms than skk294 or the ones described in the starlette issue. Here is my code:

import asyncio
from fastapi import FastAPI, BackgroundTasks, Request

app = FastAPI()

@app.get("/")
def endpoint(bg: BackgroundTasks):
	bg.add_task(f)

async def f():
	print('starting background task')
	await asyncio.sleep(1)
	print('finished background task')

@app.middleware("http")
async def db_session_middleware(request: Request, call_next):
    print('starting middleware')
    response = await call_next(request)
    print('finishing middleware')
    return response

Python 3.10.4, Fastapi 0.78.0, Uvicorn 0.17.6

The symptom is that the background task hangs as soon as it encounters await asyncio.sleep. Future requests will go the same way: I see “finishing middleware”, and “starting background task”, but never “finished background task”. If I remove the middleware, it works.

Interestingly this only happens with asyncio: if I replace asyncio.sleep with time.sleep (and remove the await), it works even with the middleware.