cgroup: make target cgroup threaded if needed

[giuseppe]

if moving a process fails with EOPNOTSUPP, then change the target
cgroup type to threaded and attempt the migration again.

Closes: #923

Signed-off-by: Giuseppe Scrivano [email protected]

[PavelSosin-320]

Maybe, it is true also in the Gnome workstation scenario where resources are also managed at the upper level, i.e. rootless session level using systemd.

[skepticoitusInteruptus]

Thanks for sparing the time to look into this fellahs (@giuseppe, @flouthoc, @PavelSosin-320).

I'm not qualified to comment on any implementation details (I'm a Java dude; learning Go is on my TODO list).

But just as a general observation @giuseppe , have you considered writing some kind of "unit test"?

From the Done! outcome you listed, I'm not 100% certain whether or not my expectations are on the same page as what this PR is implementing(?)

Do you think that some kind of test(s) that setup the preconditions and expectations/assertions I described in #923 would increase confidence that the original issue, as described, is being addressed?

[flouthoc]

@giuseppe Could we mere this or are we still waiting for discussion here #923 following PR also contains commit c44937b which we also need to unblock CI

[giuseppe]

let's merge, we can improve it later

[flouthoc]

LGTM