Eager task factory segfaults when hosting and calling an API from the same event loop

[irgolic]

Crash report

What happened?

Love eager task factory, but it's crashing one of my tests. I'm using uvicorn and fastapi to host an API, and testing it with httpx running in the same event loop. This reliably produces a segfault.

It seems to only happen with streaming responses.

Here's a minimal code example:

import asyncio

import httpx
import uvicorn
from fastapi import FastAPI
from starlette.responses import StreamingResponse


async def main():
    loop = asyncio.get_running_loop()
    loop.set_task_factory(asyncio.eager_task_factory)

    app = FastAPI()

    @app.get("/")
    async def get():
        async def _():
            yield "1"

        return StreamingResponse(
            _(),
        )

    server = uvicorn.Server(
        uvicorn.Config(
            app,
            host="0.0.0.0",
            port=8080,
        )
    )
    asyncio.create_task(server.serve())

    client = httpx.AsyncClient()
    await client.get("http://localhost:8080")


if __name__ == "__main__":
    asyncio.run(main())

CPython versions tested on:

3.12

Operating systems tested on:

macOS

Output from running 'python -VV' on the command line:

Python 3.12.3 | packaged by conda-forge | (main, Apr 15 2024, 18:35:20) [Clang 16.0.6 ]

Linked PRs

• gh-122332: Fix missing NULL check in asyncio.Task.get_coro #122338
• [3.13] gh-122332: Fix missing NULL check in asyncio.Task.get_coro (GH-122338) #122344
• [3.12] gh-122332: Fix missing NULL check in asyncio.Task.get_coro (GH-122338) #122345

[ZeroIntensity]

I was able to reproduce this. The segfault comes from _asyncio_Task_get_coro_impl: see here, self->task_coro is NULL.

[ZeroIntensity]

Ah, I think this is just a missing NULL check. Based on the docs, this should be able to return None. This needs to be reflected in typeshed as well. I'll submit a PR for this.

[ZeroIntensity]

FWIW, here's a reproducer that does not need third party packages:

import asyncio


async def main():
    loop = asyncio.get_running_loop()
    loop.set_task_factory(asyncio.eager_task_factory)

    async def foo(): ...

    task = asyncio.create_task(foo())
    await task
    print(task.get_coro())


asyncio.run(main())

[irgolic]

Nice, thanks for the fix