kill_pg could use cgroups if available

[pzmarzly]

Killing a process tree is a hard problem. In dinit, it is currently done via process groups:

dinit/src/baseproc-service.cc

Lines 438 to 453 in 712e1fa

	void base_process_service::kill_pg(int signo) noexcept
	{
	if (onstart_flags.signal_process_only) {
	bp_sys::kill(pid, signo);
	}
	else {
	pid_t pgid = bp_sys::getpgid(pid);
	if (pgid == -1) {
	// On some OSes (eg OpenBSD) we aren't allowed to get the pgid of a process in a different
	// session. If the process is in a different session, however, it must be a process group
	// leader and the pgid must equal the process id.
	pgid = pid;
	}
	bp_sys::kill(-pgid, signo);
	}
	}

Linux 5.14 added cgroup.kill, which can be used to atomically SIGKILL entire cgroup: https://lwn.net/Articles/855049/

Could dinit also use it, if compiled with cgroup support enabled?

[davmac314]

Yes, at some point there should be consideration of this. The cgroups support in general needs to be enhanced and there is already a ticket for that in #397

[davmac314]

(However, this will be post-1.0).