Adding a middleware causes background tasks cancellation

[csuriano23]

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Example Code

import asyncio
import time
from fastapi import BackgroundTasks, FastAPI

app = FastAPI()


@app.middleware("http")
async def pass_all_middleware(request, call_next):
    return await call_next(request)


async def test_bg():
    try:
        print("BG TASK STARTED")
        await asyncio.sleep(5)
        print("BG TASK ENDED")
    except BaseException as be:
        print("BG TASK NOT ENDED CORRECTLY", type(be))


@app.get("/hello")
async def root(background_tasks: BackgroundTasks):
    background_tasks.add_task(test_bg)
    return {"message": "hello"}

Description

Executing the sample code causes an unexpected (and apparently unrelated) asyncio CancelledError on long running background tasks.

Sample output running on uvicorn and calling GET on /hello endpoint:

INFO:     Started server process [95599]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:57617 - "GET /hello HTTP/1.1" 200 OK
BG TASK STARTED
BG TASK NOT ENDED CORRECTLY <class 'asyncio.exceptions.CancelledError'>

The cause seems to be the introduction of a middleware; when removing the middleware fragment everything get fine:

INFO:     Started server process [96777]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:57674 - "GET /hello HTTP/1.1" 200 OK
BG TASK STARTED
BG TASK ENDED

Operating System

macOS Ventura

Operating System Details

Darwin ARM64 M1

FastAPI Version

0.86.0

Python Version

Python 3.10.8

Additional Context

No response

[odiseo0]

I believe this is an issue with BaseHTTPMiddleware from Starlette

See BaseHTTPMiddleware and this issue

Possible solution:
Use a pure ASGI middleware

[csuriano23]

Thank you. It seems that the issue has been fixed in starlette 0.21, so I'm looking forward to have the release with this #5471

[csuriano23]

I think this can be closed after latest FastAPI release. Thank you

[tiangolo]

Thanks for the help here @odiseo0 ! 👏 🙇

Thanks for reporting back and closing the issue @csuriano23 👍