WHy mkosi box for unit tests ?

[ssooffiiaannee]

systemd 260 (260 ~ devel)
OS : Linux FRSMI25-BPYTOP 6.14.0-37-generic #37~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Nov 20 10:25:38 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

I havef tried running the systemd unit test :

$ mkosi -f genkey                                            # Generate signing keys once.
$ mkosi -f box -- meson setup -Dbpf-framework=disabled build # bpftool detection inside mkosi box is broken on Ubuntu Noble and older
$ mkosi -f box -- meson compile -C build
$ mkosi -f box -- build/systemctl --version
$ mkosi -f box -- meson test -C build --print-errorlogs -q   # Run the unit tests

According to mkosi.md:

box Run arbitrary commands inside of the same environment used to execute other verbs such as boot, shell, vm and more. This means /usr will be replaced by /usr from the tools tree if one is used while everything else will remain in place.

SO the tools tree is from my build folder ? and the environment is my host root fs without the /usr folder ? Why not run the unit tests inside a full VM for better reproducibility ?

$ mkosi box -- which systemctl
~/systemd/build/systemctl

[ssooffiiaannee]

I should have posted this in systemd issues or discussion.

[daandemeyer]

A VM is too much overhead for a simple unit test.

mkosi box exists to allow development without having to install all build dependencies on your host system. You might not be able or want to install the proper deps on your host system and mkosi box allows you to avoid that entirely.

[ssooffiiaannee]

Thanks.
I asked that because there was one failed unit test : test-cgroup-util.
After investigation, I found that the unit test expects systemd running on v2 control group, and could not find the path to /sys/fs/cgroup/init.scope file, because /sys/fs/cgroup is borrowed from the host environment with systemd running in hybrid and not v2 cgroup mode (/sys/fs/cgroup/unified, no init.scope).

[daandemeyer]

cgroupv1 support has been removed upstream, we do not support systems anymore that do not run cgroupv2

[ssooffiiaannee]

I still find it confusing that sandboxed unit tests still depend on the hosts cgroup mode.

[daandemeyer]

I still find it confusing that sandboxed unit tests still depend on the hosts cgroup mode.

The mkosi box sandbox isn't really a traditional sandbox. We don't try to hide the host, we mainly just want to plug in our own /usr. All the other stuff comes from the host. To have an actual proper sandbox you'd need a container.