What will I do next? cgroupfs \o/

With the ifupdown fixes that I published last week I actually reached my initial goal, that is to make Debian/Hurd boot using sysvinit and the initscripts provided by Debian. So on Monday we were discussing in #hurd what I could do next. Michael Banck suggested that I should port Upstart, but we agreed to do something different instead for two reasons:

1. Upstart and systemd are somewhat competing to be the default init system for Debian, and we felt it might be inappropriate to get involved with this question as porting Upstart to Hurd would probably also enable it to be used on FreeBSD. The Upstart folks could then point out that Upstart is more portable because it runs on all kernels used by Debian.
2. Upstart uses ptrace(2) to track child processes of servers it monitors. Obviously this is kind of a hack, and it was conjectured that Upstart would eventually use cgroups to do that. Also, the Hurd lacks support for ptrace(2) (that is most likely by choice by the way, ptrace(2) is not a nice interface and the Hurd (Mach actually) has much nicer interfaces to implement a debugger).

So we decided that no matter how the struggle between Upstart and systemd turns out, the Hurd would eventually need to support cgroups. So I started to write a cgroupfs translator, it is in its early stages but it already looks and acts a lot like Linux' cgroups:

% settrans -ac cg ./cgroupfs --release-agent=foobar
% ls cg
release_agent  tasks
% tail -n3 cg/tasks
11395
12869
1266
% mkdir cg/foo
% echo 1266 >> cg/foo/tasks
% tail -n3 cg/tasks cg/foo/tasks
==> cg/tasks <==
215
11395
12869

==> cg/foo/tasks <==
1266

To make this fully functional I will have to modify /hurd/proc and most likely also GNU Mach, but on the bright side this will help make subhurds (Hurds native, by-design-for-free-and-without-overhead container like functionality) work better and more securely (among other things this could enable non-root users to start subhurds). I will also look into porting libcg (I have a hacky patch series ready) so that we can actually test the cgroupfs translator. All current users of the cgroup interface are very Linux specific (surprise!), and libcg looks like the easiest one to port. And they do have a test suite that could help me improve the cgroupfs translator.