nixos/qemu-guest-agent: fix start service

[Izorkin]

Motivation for this change

Fix start qemu-guest-agent service

cc @Mic92 @blitz @flokli

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[blitz]

Looks ok to me from the code changes. I don't have a system to test this, though.

[flokli]

I'd prefer if we could fix this while building qemu itself, not when invoking the qemu-guest-agent binary. Using a path inside the nix store can't ever work.

We also might be able to test this in a VM test, by waiting for the systemd unit to have succeeded in starting up.

The test instrumentation sets services.qemuGuest.enable, so the unit should be configured in every test VM.

[Izorkin]

Don't know how to fix qemu build.

[flokli]

This path is assembled in qga/main.c, which calls to qemu_get_local_state_pathname (from util/oslib-posix.c).

This uses the CONFIG_QEMU_LOCALSTATEDIR define, which is set in meson.build.

For some reason, this is set to get_option('prefix') / get_option('localstatedir') (which doesn't work on NixOS).

I'm not sure how it works on other distros, but we might just want to ship a downstream patch in nixpkgs, that changes this to another path. (and do some digging upstream about why it's done that way).

[Izorkin]

@flokli with add to configureFlags option "--localstatedir=/var" error build qemu if activated nix.useSandbox = true;

[Patryk27]

FWIW, I've just got bit by qemu-agent not starting, and this patch helped.

As for the approach taken - as far as I know, going with the --statedir-esque kind of parameters is a pretty common practice; maybe instead of trying to work-around this issue by patching qga/main.c (which seems kinda brittle), we could apply the easy, low-hanging-fruit as in this merge request?

[AluisioASG]

Is this needed to avoid shipping a broken service with 21.05?