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run_process: Document pitfalls re/ Ctrl+C and not creating a new process group #3300
Description
I know the docs already mention process groups;
However, I had no idea about the existence of background and foreground groups, and didn't know that a grandchild process could take control of Ctrl+C handling if it becomes a foreground process and pushes my process into background (?). So I spent quite a lot of time trying to figure out why Ctrl+C wasn't reaching my process at all.
#3273 is relevant, but even for non-shell invocations of run_process it's still possible to lose control over Ctrl+C unless you use smth like "start_new_session".
Activity
A5rocks commentedon Jul 14, 2025
You can use
pstreeto debug what is a parent to what!Would it have been valuable to you to have any of the following?:
process_group=0for non-shell processes too. (I don't know how to do something nicely on Windows)Does
start_new_sessionhelp? My impression was that it's mostly for setting up a controlling terminal but I also don't really know my way around Unix :-)neongreen commentedon Jul 15, 2025
start_new_sessionhelps, yeah, but now that i've read the docs I wonder if maybe I should've usedprocess_group=0instead.I used
procsto show the tree, but I think my children are my children regardless of what process group they're in? Not sureThis would've helped for my usecase but I also don't know my way around Unix (apparently 😭) so I can't say if it's a good idea in general or not
The problem is that I had no idea process groups were even relevant for Ctrl+C handling, so having perfect awareness of the process topology would not have helped me, I suppose
A5rocks commentedon Sep 26, 2025
OK, so I actually ran into https://github.com/oconnor663/duct.py/blob/master/gotchas.md and it suggests that cgroups (shock! horror!) are a good default. Ie just make a cgroup for every spawned subprocess and then you can atomically send a signal to that.
(And that "job groups" exist on Windows. So it would work... except on macos/bsd stuff...)
I would be happy with that default!
Though now that I'm rereading this issue I think you were running into something else... oh well.
nightcityblade commentedon Apr 14, 2026
Hi, I'd like to work on this. I'll submit a PR shortly.
Madhur-7568 commentedon Jul 14, 2026
Hi, I'd like to work on this , can i make a pr.
A5rocks commentedon Jul 15, 2026
Of course you can make a PR. However, it's a bit of a challenge to write docs that fix this, IIRC.
Madhur-7568 commentedon Jul 25, 2026
I'm thinking of expanding the existing Ctrl+C note to mention this behaviour and explain when process_group or start_new_session might be useful. Does that sound like the right direction?
Madhur-7568 commentedon Jul 31, 2026
Hi! Just following up on my previous message. Please let me know when you have a chance.
A5rocks commentedon Jul 31, 2026
I kind of forgot all the context for this issue and will only try to remember if there's actually a patch to the docs being proposed :P
All I remember thinking is that I don't really know the optimal resolution for this issue.