multiprocessing/joblib doesn't work in uvicorn

[qwertyforce]

Checklist

• The bug is reproducible against the latest release or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

app freezes when trying to use multiprocessing.Pool or joblib (joblib/joblib#1250)

Steps to reproduce the bug

import uvicorn
import multiprocessing
if __name__ == '__main__':
    uvicorn.run('uvicorn_test:app', host='127.0.0.1', port=7777, log_level="info")

def calculate(func, args):
    result = func(*args)
    return '%s says that %s%s = %s' % (
        multiprocessing.current_process().name,
        func.__name__, args, result
        )

def calculatestar(args):
    return calculate(*args)

def mul(a, b):
    return a * b

def plus(a, b):
    return a + b

if __name__ == 'uvicorn_test':
    with multiprocessing.Pool(1) as pool:
        print(pool._processes)
        TASKS = [(mul, (i, 7)) for i in range(10)] + \
                [(plus, (i, 8)) for i in range(10)]
        imap_it = pool.imap(calculatestar, TASKS)
        for x in imap_it:
            print('\t', x)

Expected behavior

print some text

Actual behavior

app hangs

Debugging material

No response

Environment

Running uvicorn 0.17.0 with CPython 3.8.10 on Windows
joblib doesnt work on: linux, windows; python 3.7 3.8 3.9

Additional context

No response

[Kludex]

How do I run the example?

[qwertyforce]

@Kludex

1. copy code into uvicorn_test.py

2. run

3. you will see this
   

4. comment out

if __name__ == '__main__':
    uvicorn.run('uvicorn_test:app', host='127.0.0.1', port=7777, log_level="info")

in if __name__ == 'uvicorn_test': change uvicorn_test to __main__so code becomes like this

import uvicorn
import multiprocessing
# if __name__ == '__main__':
#     uvicorn.run('uvicorn_test:app', host='127.0.0.1', port=7777, log_level="info")

def calculate(func, args):
    result = func(*args)
    return '%s says that %s%s = %s' % (
        multiprocessing.current_process().name,
        func.__name__, args, result
        )

def calculatestar(args):
    return calculate(*args)

def mul(a, b):
    return a * b

def plus(a, b):
    return a + b

if __name__ == '__main__':
    with multiprocessing.Pool(1) as pool:
        print(pool._processes)
        TASKS = [(mul, (i, 7)) for i in range(10)] + \
                [(plus, (i, 8)) for i in range(10)]
        imap_it = pool.imap(calculatestar, TASKS)
        for x in imap_it:
            print('\t', x)

5. run file again
   
   it's working

[Kludex]

Where's the app?

On Ubuntu 20.04:

❯ python uvicorn_test.py 
ERROR:    Error loading ASGI app. Attribute "app" not found in module "uvicorn_test".

[qwertyforce]

On Debian 10 it's working. I use uvicorn[standard] 0.17.0.post1.
idk you can add this to existing code

from fastapi import FastAPI
app = FastAPI()

but it should work even without app.

[tomchristie]

@Kludex - I think our general policy ought to be to push these kinda things into "discussions" first. (Probably?) Do you have permissions to do that ATM?

[Kludex]

I do. Sure. 👍