middleware breaks behavior of background task

[jkh911208]

First check

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Example

import asyncio
import logging
import time

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from starlette.background import BackgroundTasks

app = FastAPI()


@app.get("/")
async def read_root():
    response_object = {"Hello": "World"}
    tasks = BackgroundTasks()
    tasks.add_task(bg_task)
    return JSONResponse(response_object, 202, background=tasks)


async def bg_task():
    logging.error("test1")
    await asyncio.sleep(10)
    logging.error("test2")


@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.time()
    response = await call_next(request)
    response.headers["X-Process-Time"] = str(time.time() - start_time)
    response.headers["address"] = request.client.host
    return response

if __name__ == "__main__":
    import uvicorn
    uvicorn.run("main:app", host="0.0.0.0", port=8000,
                log_level="debug", reload=True)

Description

run the above code, make a request to "/"
you will see

which logging is happening before return the object

but if you make only one request at a time this is fine, but if you make multiple request while background task is running

problem #1 the second request start after first background task finishes
problem #2 when background task of second request finishes, it throw error

Environment

• OS: [e.g. Linux / Windows / macOS]: macos and debian
• FastAPI Version [e.g. 0.3.0]: 0.61.1
• Python version: 3.8.5

Additional context

if i remove middleware code it works as expected

all requests are handled and all background tasks are working fine no exception

[ycd]

I believe this is related to Starlette Issue 919: Background tasks don't work with middleware that subclasses BaseHTTPMiddleware

[jkh911208]

thank you @ycd seems like work is still in progress

[Kludex]

@jkh911208 Could you close this issue, please?

[yingyingPengSAP]

For those who are searching for how to call background tasks under FastAPI middleware, here is quick solution:

from starlette.background import BackgroundTask
from somewhere import functionA

@app.middleware("http")
async def middleware(request: Request, call_next):
........(do something)
response.background = BackgroundTask(functionA, arg)
return response

You can add a time.sleep(10) in function A to see the result better.
For tasks running in background, it will return the results of function A
very quickly and will not be affected by time.sleep().

Hope this helps someone!

[santigandolfo]

Hi @jkh911208 @Kludex , why is this issue Closed? Cause I'm trying to use background tasks and a middleware at the same time, and if I send a request to the endpoint through Postman, I receive my response but the request time doesn't finish until the background task does. But if I remove the middleware then it works as expected.

[Kludex]

What is the version of your FastAPI?

Oh sorry, yeah, this is still a thing. encode/starlette#1441

[santigandolfo]

What is the version of your FastAPI?

Oh sorry, yeah, this is still a thing. encode/starlette#1441

I'm on v0.75.2
Do you suggest a workaround until that PR is merged in Starlette and FastAPI bumps it's requirements?

[Kludex]

Yes, don't use BaseHTTPMiddleware.

[santigandolfo]

Yes, don't use BaseHTTPMiddleware.

In that case I couldn't add general before and after request actions, right?

[Kludex]

No, I'm saying to not use that class. You can have middlewares: https://pgjones.dev/blog/how-to-write-asgi-middleware-2021