asyncio.create_subprocess() cancellation

[socketpair]

Documentation

It's not documented, that .cancel() kills background process with kill(). At least on Linux:

from asyncio import Task, create_subprocess_exec, run, sleep
async def main():
    t = Task(create_subprocess_exec('sleep', '100'))
    await sleep(1)
    t.cancel()
    await t
run(main())

strace -t -ff -e clone,fork,vfork,execve,kill,wait4 /usr/bin/python3 test.py 
22:19:20 execve("/usr/bin/python3", ["/usr/bin/python3"], 0x7ffe4ff89598 /* 50 vars */) = 0
22:19:20 vfork(strace: Process 479095 attached
 <unfinished ...>
[pid 479095] 22:19:20 execve("/usr/bin/sleep", ["sleep", "100"], 0x7ffde73e6110 /* 50 vars */ <unfinished ...>
[pid 479094] 22:19:20 <... vfork resumed>) = 479095
[pid 479095] 22:19:20 <... execve resumed>) = 0
[pid 479094] 22:19:21 wait4(479095, 0x7ffde73e58d4, WNOHANG, NULL) = 0
[pid 479094] 22:19:21 wait4(479095, 0x7ffde73e5954, WNOHANG, NULL) = 0
[pid 479094] 22:19:21 kill(479095, SIGKILL) = 0
[pid 479094] 22:19:21 wait4(479095,  <unfinished ...>
[pid 479095] 22:19:21 +++ killed by SIGKILL +++
22:19:21 <... wait4 resumed>[{WIFSIGNALED(s) && WTERMSIG(s) == SIGKILL}], WNOHANG, NULL) = 479095
22:19:21 --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=479095, si_uid=1000, si_status=SIGKILL, si_utime=0, si_stime=0} ---
22:19:21 +++ exited with 0 +++

Linked PRs

• gh-118516: clarify that subprocess are automatically killed if transport gets garbage collected #140997
• [3.14] gh-118516: clarify that subprocess are automatically killed if transport gets garbage collected (GH-140997) #141110
• [3.13] gh-118516: clarify that subprocess are automatically killed if transport gets garbage collected (GH-140997) #141111

[socketpair]

I think here: https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.subprocess_exec
and here: https://docs.python.org/3/library/asyncio-subprocess.html#asyncio.create_subprocess_exec

[kumaraditya303]

Yes, this is expected as BaseSubprocessTransport's close method kills the process if it is still alive. It is unrelated to cancellation, when the last ref to process is dropped the process is closed via __del__.