syncthing: Fix systemd service

[jokogr]

Motivation for this change

On some systems I tend to run syncthing only as a user, e.g.:

  services.syncthing = {
    enable = true;
    systemService = false;
  };

I have noticed recently that the syncthing service was missing. It appears that due to a refactoring, the syncthing package has now multiple outputs (bin and out). This PR:

1. Fixes the executable paths in the systemd service files to the bin output.
2. Includes both outputs in systemd.packages (bin to have the executables in /nix/store and out to include properly the systemd files - the latter was the reason that the syncthing service was missing)
3. Stops creating service users if syncthing is not a system service.

I am still unsure whether to include syncthing.bin in systemd.packages, let me know if we can do it in another way, e.g. include it in system.extraDependencies.

Pinging @Mic92 since he refactored the syncthing package to use buildGoPackage in 643fabf.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[Infinisil]

What's the problem if pkgs.syncthing.bin isn't included here?

[jokogr]

Fair question, initially it was pkgs.syncthing which meant pkgs.syncthing.bin. This was a problem, since the bin output does not have the systemd service files.

By using pkgs.syncthing.out the reference to the binaries got lost, so I thought to include bin as a systemd.package.

[Infinisil]

Looked it up, systemd.packages (as I thought) only uses $out/etc/systemd of the packages, it only adds systemd units defined in them:

nixpkgs/nixos/modules/system/boot/systemd-lib.nix

Lines 150 to 152 in 830599d

	# Symlink all units provided listed in systemd.packages.
	for i in ${toString cfg.packages}; do
	for fn in $i/etc/systemd/${type}/* $i/lib/systemd/${type}/*; do

So there won't be any problems with removing that (and pkgs.syncthing.out = pkgs.syncthing)

[jokogr]

So there won't be any problems with removing that (and pkgs.syncthing.out = pkgs.syncthing)

I am afraid this is not correct, from my tests it is more like pkgs.syncthing.bin = pkgs.syncthing.

My impression is that buildGoPackage in pkgs/development/go-modules/generic/default.nix sets

 outputs = args.outputs or [ "bin" "out" ];

so the first output ("bin") is picked when output is not defined (a.k.a. pkgs.syncthing).

So, we have to explicitly declare the "out" output in the systemd.packages and then again we are missing a reference to the other output, which contains binaries.

[Infinisil]

Ohh, I didn't check, you're right. pkgs.syncthing.out in systemd.packages should be good then. The binaries aren't needed in systemd.packages though, so that can be removed.

[jokogr]

ok, good thing I understood out well.

Now, I agree that binaries are not needed in systemd.packages, but if I leave syncthing.bin out, then it is not referred anywhere else, so it is not installed. This is why I kept it.

Other approaches would be:

1. To include bin in environment.systemPackages.
2. To include bin in system.extraDependencies as mentioned in motivation.
3. To move the systemd files to $bin as suggested by @Mic92.

Personally I find 2 the most elegant, but it is mentioned that this option is primarily used by the installation tests. In the end, I will follow @Mic92's suggestion.

[Infinisil]

In what way does putting it in systemd.packages make it get installed? It shouldn't. If by "install" you mean having it accessible in $PATH, then environment.systemPackages is what you want (and I suspect you do). In NixOS it doesn't make sense to "install" something just to have it be built. Either it's needed (and then you refer to it where you need it and it gets available like this), or it's not needed (so you don't refer to it and it won't be available). There's no notion of installing it just to have it installed.

[Mic92]

syncthing provides systemd user units not system-wide ones.

[Mic92]

Now you can use pkgs.syncthing again. In future we should get rid of the pointless out output of go packages. We had go source code there before, which also didn't made any sense. However migrating from $bin back to $out is a bigger refactoring challenge.

[Mic92]

Can you move all outputs to $bin instead?

[Mic92]

@GrahamcOfBorg build syncthing

[rnhmjoj]

status?

[jokogr]

• Followed @Mic92's piece of advice and moved the systemd unit files to the bin output.
• Rebased to current master.
• Retested the service on my machine, it still works as intended.

[Infinisil]

Oh, can you format your last commit message with the nixos/syncthing: prefix?

[jokogr]

Amended and rebased once again :)