uvloop runs at_fork hook when running subprocess

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

首先跟踪 uvloop 对 asyncio.create_subprocess_exec 的处理，以及在 uvloop.install() 之后使用的 fork 路径，并将其与标准 asyncio 循环和 os.register_at_fork 的行为进行比较。使用 uptime 子进程重新运行提供的复现；当此次子进程启动未触发 after_in_child hook 时，即表示完成。