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uvloop runs at_fork hook when running subprocess

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#749 0 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
58/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Tech stack
python
Domain
operating-systems

Research direction

Start by tracing uvloop's handling of asyncio.create_subprocess_exec and the fork path used after uvloop.install(), comparing it with the standard asyncio loop and os.register_at_fork behavior. Re-run the provided reproduction with the uptime subprocess; done means the after_in_child hook is not triggered for this subprocess launch.

Written by the indexing model from the issue text.

Description

Speaking about os.register_at_fork python docs explicitly state:

These calls are only made if control is expected to return to the Python interpreter. A typical subprocess launch will not trigger them as the child is not going to re-enter the interpreter.

The standard asyncio loop behaves this way. But uvloop doesn't. Repro:

import asyncio
import os
   
import uvloop


def hook():
    os.write(2, b"TRIGGERED\n")
    
    
async def run():        
    process = await asyncio.create_subprocess_exec(
        "uptime",
        stdout=asyncio.subprocess.PIPE,
        stderr=asyncio.subprocess.PIPE,
        start_new_session=True
    )

    stdout, stderr = await process.communicate()
    print(f"STDOUT: {stdout.decode('utf-8').strip()}")
    

if __name__ == "__main__":
    os.register_at_fork(after_in_child=hook)

    print("asyncio")
    asyncio.run(run())
    
    print("uvloop")
    uvloop.install()
    asyncio.run(run())

Output:

asyncio
STDOUT: 16:56:42 up 35 days,  6:20,  1 user,  load average: 5.21, 4.34, 4.27
uvloop
TRIGGERED
STDOUT: 16:56:42 up 35 days,  6:20,  1 user,  load average: 5.21, 4.34, 4.27
Dominant language
Cython
Stars
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Forks
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