fastapi: FastAPI gets terminated when child multiprocessing process terminated

Describe the bug

Make a multiprocessing Process and start it. Right after terminate the process, fastapi itself(parent) terminated.

To Reproduce

Start command: /usr/local/bin/uvicorn worker.stts_api:app --host 127.0.0.1 --port 8445

Create a file with:

from fastapi import FastAPI

app = FastAPI()


@app.post('/task/run')
def task_run(task_config: TaskOptionBody):
    proc = multiprocessing.Process(
        target=task.run,
        args=(xxxx,))
    proc.start()
    return task_id

@app.get('/task/abort')
def task_abort(task_id: str):
    proc.terminate()
    return result_OK

Run task_run and while the process alive, trigger task_abort
After child process terminated then parent(fastApi) terminated as well.

Expected behavior

Parent process should not be terminated after child terminated.

Environment

OS: Linux
FastAPI Version 0.54.1
Python version 3.8.2

Additional context

I tried same code with Flask with gunicorn, it never terminated.

About this issue

Original URL
State: closed
Created 4 years ago
Reactions: 6
Comments: 23 (9 by maintainers)

Most upvoted comments

Hi, I have discovered this situation and came to the next conclusions:

Uvicorn register signals handlers and child process inherit them (but also inherit ThreadPoolExecutor and another resources)
~~You cannot set signals handlers not from the main thread~~
~~The task function will be executed in the ThreadPoolExecutor, so as I say early - you cannot change signal handlers in this function;~~

The second and third conclusions is not true, the real problem was founded and described below.

But it still possible to solve this problem (without changing FastAPI or uvicorn) - you can change start_method for multiprocessing to spawn method and your child process will be clear (without inherited signals handles, thread pools and other stuff).

@app.post('/task/run')
def task_run():
    multiprocessing.set_start_method('spawn')
    proc = multiprocessing.Process(
        target=task,
        args=(10,))
    proc.start()
    
    return proc.pid

It’s work for me (python3.7, macOS 10.15.5)

+13

Mixser on Jun 16, 2022

@victorphoenix3 It seems like your process need to have some long running code. Just try to add “time.sleep(30)” and try to abort this within the time.

I tried your code and there is no issue. (Because the subprocess already terminated…?) 1 21742 INFO: 127.0.0.1:52778 - “POST /task/run HTTP/1.1” 200 OK INFO: 127.0.0.1:52780 - “GET /task/abort?pid=21742 HTTP/1.1” 200 OK

But after I adding “sleep 30 seconds”, and the issue comes. 1 21982 INFO: 127.0.0.1:52802 - “POST /task/run HTTP/1.1” 200 OK INFO: 127.0.0.1:52804 - “GET /task/abort?pid=21982 HTTP/1.1” 200 OK INFO: Shutting down INFO: Waiting for application shutdown. INFO: Application shutdown complete. INFO: Finished server process [21973]

jongwon-yi on Jun 17, 2020

@tiangolo I think you can close this, because this is not related to the FastApi or uvicorn – this is a specific behaviour of signal handling in asyncio.

Mixser on Jun 16, 2022

To avoid RuntimeError('context has already been set') when set_spawn_method is called multiple times within a route, I moved the call into FastAPI’s startup event handler so it is only called once, as prescribed in the stdlib docs. This solved this issue for me.

import multiprocessing

...
app = FastAPI()


@app.on_event("startup")
def startup_event() -> None:
    multiprocessing.set_start_method("spawn")

johnthagen on Jun 5, 2021