os/modules/docker-ci-runner/default.nix

{
  lib,
  config,
  pkgs,
  ...
}:
with lib; let
  bootstrap = pkgs.writeScript "bootstrap.sh" ''
    #!/usr/bin/env bash

    set -e

    apt update
    apt install --yes curl git sudo xz-utils

    adduser --system --uid 999 build
    chown build /nix

    sudo -u build curl -L https://nixos.org/nix/install > install
    sudo -u build sh install

    echo "export PATH=/nix/var/nix/profiles/per-user/build/profile/bin:''$PATH" >> /etc/profile

    mkdir /etc/nix
    echo 'experimental-features = nix-command flakes' >> /etc/nix/nix.conf

    export nix_user_config_file="/home/build/.local/share/nix/trusted-settings.json"
    mkdir -p $(dirname \\$nix_user_config_file)
    echo '{"extra-experimental-features":{"nix-command flakes":true}}' > \\$nix_user_config_file
    chown -R build /home/build/

    curl -L https://github.com/drone-runners/drone-runner-exec/releases/latest/download/drone_runner_exec_linux_amd64.tar.gz | tar xz
    sudo install -t /usr/local/bin drone-runner-exec

    if [ ! -f /run/vars ]; then
      exit 1
    fi

    cp -a /run/vars /run/runtime-vars
    env | grep "DRONE" >> /run/runtime-vars

    su - -s /bin/bash build sh -c "/usr/local/bin/drone-runner-exec daemon /run/runtime-vars"
  '';
  psCfg = config.pub-solar;
  cfg = config.pub-solar.docker-ci-runner;
in {
  options.pub-solar.docker-ci-runner = {
    enable = lib.mkEnableOption "Enables a docker container running a drone exec runner as unprivileged user.";

    enableKvm = lib.mkOption {
      description = ''
        Enable kvm support.
      '';
      default = true;
      type = types.bool;
    };

    nixCacheLocation = lib.mkOption {
      description = ''
        Location of nix cache that is shared between builds
      '';
      default = "/var/lib/docker-ci-runner";
      type = types.path;
    };

    runnerEnvironment = lib.mkOption {
      description = ''
        Additional environment vars added to the vars file on container runtime
      '';
      default = {};
    };

    runnerVarsFile = lib.mkOption {
      description = ''
        Location of vars file passed to drone runner
      '';
      type = types.path;
    };
  };

  config = lib.mkIf cfg.enable {
    virtualisation = {
      docker = {
        enable = true; # sadly podman is not supported rightnow
      };

      oci-containers = {
        backend = "docker";
        containers."drone-exec-runner" = {
          image = "debian";
          autoStart = true;
          entrypoint = "bash";
          cmd = ["/bootstrap.sh"];

          volumes = [
            "${cfg.runnerVarsFile}:/run/vars"
            "${cfg.nixCacheLocation}:/nix"
            "${bootstrap}:/bootstrap.sh"
          ];

          environment = cfg.runnerEnvironment;

          extraOptions = lib.mkIf cfg.enableKvm ["--device=/dev/kvm"];
        };
      };
    };
    # Fix container not stopping correctly and holding the system 120s upon
    # shutdown / reboot
    systemd.services.docker-drone-exec-runner.preStop = ''
      docker stop drone-exec-runner
    '';
  };
}