Add deliver_cancel parameter and start support to run_process

[gschaffner]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

Trio

1. has a deliver_cancel parameter and nursery.start support in run_process
2. changed their examples to suggest that folks use process = await nursery.start(run_process, ...) instead of open_process.
3. moved open_process to lowlevel
4. removed Process.aclose

Use case

• process = await nursery.start(trio.run_process, ...) actually enforces structured concurrency—you can't accidentally leave an orphan process¹
• open_process does not enforce structured concurrency, hence it should be in lowlevel. while you can make it enforce structured concurrency by doing async with await trio.lowlevel.open_process() as process, process = await trio.lowlevel.open_process() by itself does not use structured concurrency and is basically fork() without a finally: wait(pid) to follow it. hence i lean toward agreeing that open_process should be in lowlevel.

i'd like to propose doing (1) and (2) in AnyIO: adding deliver_cancel support and TaskGroup.start support to run_process. i actually ended implementing this some months ago in a downstream AnyIO application at work, because upon cancellation we needed to send the child a gentle SIGINT first instead of reaching straight for SIGKILL and causing data loss. i'd be happy to move that implementation upstream if it seems reasonable.

if proposals (1) and (2) are accepted, AnyIO could also consider doing (3) and/or (4). they also make sense to me, as mentioned above—put open_process in lowlevel so users think twice before shooting themselves in the foot with the gun-that-orphans-processes.

thoughts?

Footnotes

1. except if the parent gets SIGKILL/TerminateProcess and can't run any except/finally/__exit__/__aexit__/atexit code. ↩