Enable direct installation with native systemd

[htngr]

With this fix wsl.nativeSystemd = true works in a fresh installation (should fix #156 (comment))

Minimal working configuration.nix:

wsl = {
  enable = true;
  nativeSystemd = true;
};

Note that wsl should be restarted after import (wsl --shutdown)

[nzbr]

While this makes it possible to import a tarball where wsl.nativeSystemd is enabled, it will not work properly on the first start (it requires a manual restart of the distro for systemd to start). The better solution would be to extend the systemd shim so it is able to run the installer before handing over to systemd. And we shouldn't set a root shell. People want to be able to use different shells

[htngr]

Alright, seems reasonable. I've been trying to get the installer to work with systemd, but this seems to be only possible with some ugly hacks, because

1. we can't switch between non-systemd WSL (boot.systemd = false in wsl.conf) and systemd-enabled WSL without restarting WSL. Therefore the installer needs to be systemd-enabled
2. when starting the systemd-enabled installer (put the installer script in /sbin/init), WSL expects systemd to run at latest in 10 seconds. The extraction of the tarball, the activation and the cleaning of the nix store must be finished within this timeframe, or else WSL just kills the current process and moves on in the installer script. This process probably is tar xf, so the extraction stops and the rest of the script doesn't work.

The good news is, the installer is unneccessary if we add extraArgs = "--hard-dereference" to make-system-tarball. I've only added a script (which calls /nix/var/nix/profiles/system/activate and then systemd-shim) to /sbin/init which is only called on first start and is then overwritten by the activation scripts.

One problem which remains is that for the init script to work, user.default must be root (I just override this in the initial wsl.conf). But this means that after the install the user gets a root shell and even if user.default is reset to nixos by the NixOS activation, WSL must be restarted for the change to take effect.
Edit: works regardless of user.default

Another minor thing is that user@0.service fails, so systemctl status shows degraded. After systemctl restart user@0.service everything is working as expected:
Edit: this is likely another issue because it also happens after WSL restart and for other users as well (e.g. user@1000.service)

Apr 13 23:46:15 nixos systemd[1]: Starting User Manager for UID 0...
Apr 13 23:46:15 nixos systemd[1]: user@0.service: Main process exited, code=exited, status=219/CGROUP
Apr 13 23:46:15 nixos systemd[1]: user@0.service: Failed with result 'exit-code'.
Apr 13 23:46:15 nixos systemd[1]: Failed to start User Manager for UID 0.

[htngr]

Also, would it make sense to add the workaround from microsoft/WSL#8879 (comment) to properly display the service hierarchy with systemctl status?

[K900]

I'd rather wait for this to be fixed upstream, we don't want to rebuild systemd.

[nzbr]

Agreed, we shouldn't fix things that are broken on Microsoft's end. The goal is that NixOS behaves like any other WSL distro

[K900]

Do we even need an installer at all? Maybe we should just build everything into the tarball, and then have two tarballs, one for native systemd and one for syschdemd (and then drop syschdemd later).

[htngr]

Do we even need an installer at all? Maybe we should just build everything into the tarball, and then have two tarballs, one for native systemd and one for syschdemd (and then drop syschdemd later).

This PR removes the need for an installer with native systemd.

Some time ago I've also successfully installed NixOS-WSL (syschdemd, no installer) by setting extraArgs = "--hard-dereference"; in build-tarball.nix. But I don't know if this still works

[TeofilC]

FWIW I'm pretty sure "--hard-dereference" works. I've used it successfully in the past. Also I think the upstream bug in libarchive might have been fixed and the version updated on windows. My system has bsdtar 3.5.2 - libarchive 3.5.2 zlib/1.2.5.f-ipp bz2lib/1.0.6, which I think has the fix for libarchive/libarchive#1044

[K900]

Gonna close this in favor of #295 which removes the installer hackery entirely.