Contextvars are not reset between requests if the request contains multipart form data

[Kludex]

Discussed in #2312

^{Originally posted by simon-sk October 26, 2023}
Hi Starlette team,

we are experiencing an issue with Starlette where contextvars set in a middleware are leaking between requests if the request contains multi-part form data and the client session is reused.

I constructed a minimal example which sets a context var in a middleware and resets it after the app has been called. It also raises an exception if the var is already set:

import contextvars

from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.responses import JSONResponse
from starlette.routing import Route

var = contextvars.ContextVar("var", default=None)


class ASGIMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if scope["type"] != "http":
            return await self.app(scope, receive, send)

        if var.get() is not None:
            raise ValueError(f"var is already set {var.get}")

        token = var.set("TEST")

        await self.app(scope, receive, send)

        var.reset(token)


async def route(request):
    return JSONResponse({"hello": "world"})


app = Starlette(
    middleware=[Middleware(ASGIMiddleware)],
    routes=[Route('/', route, methods=["POST"])]
)


if __name__ == "__main__":
    import uvicorn

    uvicorn.run(
        "app.main_starlette:app",
        host="0.0.0.0",
        port=8001,
    )

Then, I execute a second script which calls this endpoint using a httpx client and includes a files object:

import httpx

if __name__ == "__main__":
    url = "http://localhost:8001/"
    files = {'file': open('./test.yaml', 'rb')}

    with httpx.Client() as client:
        for i in range(2):
            r = client.post(url, timeout=None, files=files)
            assert r.status_code == 200

This results in a server error due to the fact that the variable is already set (sometimes it is necessary to run the client script multiple times as the issue does not always occur).

If I execute the same script without multipart data, it works without issues:

...
r = client.post(url, timeout=None)
...

We have also experienced this issue without sending multipart form data for requests within the same client session but it occurs significantly less often and unfortunately I am not able to reproduce it consistently.

We experienced this issue with Python 3.9 and 3.10, Starlette v0.31.1, and uvicorn v0.23.2.

[Kludex]

I can't reproduce on the latest versions of the packages, and considering Python 3.9 and 3.10.

If you can create a repository, and show me in the CI how this fails, I can continue helping.

[simon-sk]

@Kludex I created a repo here that reproduces the issue in this action run.

The issue does not always occur on the first request, so I added a loop that sends multiple requests in a row.

[Kludex]

@Kludex I created a repo here that reproduces the issue in this action run.

The issue does not always occur on the first request, so I added a loop that sends multiple requests in a row.

I can reproduce your MRE using uvicorn running with the AsyncIO event loop on Python 3.9. I cannot reproduce with the latest Python version, and I also can't reproduce with uvloop.

[9dogs]

I'm having the same issue with Python 3.11.9 and starlette==0.38.6 (transient dep from the FastAPI==0.115.0).

In my case, I'm doing simple JSON POST, but I only notice it on "high enough" loads.
Installing the uvloop fixes the issue indeed...