nixos/swapspace: init module

[phanirithvij]

Adds a swapspace nixos module.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

cc @Luflosi

[Luflosi]

This already looks very good and I only have a couple small improvement suggestions.

[Luflosi]

There was a previous attempt in #88093 but the original author seems to have lost interest.
I compared the two and think this PR is a strict improvement over the previous one.

[Luflosi]

I think extraArgs should be a list of strings instead of a single string. This seems to be what most NixOS modules do. It allows setting different options in different places without overriding previous ones.

[Luflosi]

Your latest changes removed

      after = [
        "local-fs.target"
        "swap.target"
      ];
      requires = [
        "local-fs.target"
        "swap.target"
      ];
      wantedBy = [ "multi-user.target" ];
      documentation = [ "man:swapspace(8)" ];
      description = "Swapspace, a dynamic swap space manager";

While it is true, that those changes are in the upstream unit config and thus will be readable by systemd, the same is not true for Nix itself. Nix can't read the upstream unit config during evaluation. Having information such as description available for Nix to read, could allow tools to provide useful information about NixOS modules without having to compile or download any software. If I remember correctly, wantedBy = ... is even required for the unit to be installed but I could be wrong on that. Feel free to test.
As a rule of thumb, I remove everything from systemd.services.myCoolService.serviceConfig, which is already present in the upstream unit config, while leaving everything else in systemd.services.myCoolService as it is. Unfortunately I don't know how to check which values are actually used by other NixOS modules.

[phanirithvij]

Sorry for the force push spam, I have another branch in my fork to test with my system flake, mistakenly pushed to this a few times before testing.

I've re-added those I've removed, and also fixed serviceConfig.ExecStart https://discourse.nixos.org/t/clear-list-nix-syntax-or/20580

[Luflosi]

The code looks good. There is one small non-blocking nit-pick that can be safely ignored.

[Luflosi]

Sorry for the force push spam, I have another branch in my fork to test with my system flake, mistakenly pushed to this a few times before testing.

No worries.

and also fixed serviceConfig.ExecStart

Good catch! I forgot that this was needed.

[r-vdp]

That being said, I'm also not sure that we have the unit file in the right location in the derivation output. I thought they had to be in lib, I'm not sure if they are also picked up correctly from etc.

I just had a look at the source, and lib/ and etc/ are treated in the same way, so I think this is fine.

[r-vdp]

I didn't run the module, but the diff LGTM.

[r-vdp]

@phanirithvij maybe a nixos VM test would be useful? To check that everything starts correctly.

[r-vdp]

@ofborg test swapspace

[phanirithvij]

@ofborg test swapspace

[phanirithvij]

Tested locally tons of times

nix-build -A nixosTests.swapspace; for i in {1..100}; do echo $i > run; nix-build -A nixosTests.swapspace --check || break; done

It is not flaky anymore on my x86_64-linux. And runs on aarch64-linux, So it is good to go.

---

The test can be improved later.

I've tried for a while to simplify the test but it was getting killed randomly due to oom (very flaky).
So if someone is able to write a better/leaner test, I think it can follow in a later PR.

Requesting review again @r-vdp @Luflosi @Pandapip1

[github-actions]

Backport failed for release-24.05, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-24.05
git worktree add -d .worktree/backport-348588-to-release-24.05 origin/release-24.05
cd .worktree/backport-348588-to-release-24.05
git switch --create backport-348588-to-release-24.05
git cherry-pick -x 80ea320fe72bd16d9add8a34a6b6aa3880bb9f58 e4c898c8071c5333cc283620dd14a5638a09957e