Containers as kernel objects — again

bubblewrap is an example of a program that forks into a container, turns the forked child into pid 1/the reaper for the container, and forks again to run the useful content of the container. It's the container-runner for Flatpak, among others (analogous to the role of runc in Docker), and Flatpak apps all run as pid 2 inside the container, unless they fork again.

The actual reaper process is very simple: it just calls wait() in a loop. The complicated parts of something like systemd (or even sysvinit) are the parts that set up and run all the services, not the part that reaps processes.