{ config
, lib
, pkgs

, cfg ? config.services.github-runner
, svcName

, systemdDir ? "${svcName}/${cfg.name}"
  # %t: Runtime directory root (usually /run); see systemd.unit(5)
, runtimeDir ? "%t/${systemdDir}"
  # %S: State directory root (usually /var/lib); see systemd.unit(5)
, stateDir ? "%S/${systemdDir}"
  # %L: Log directory root (usually /var/log); see systemd.unit(5)
, logsDir ? "%L/${systemdDir}"
  # Name of file stored in service state directory
, currentConfigTokenFilename ? ".current-token"

, ...
}:

with lib;

let
  workDir = if cfg.workDir == null then runtimeDir else cfg.workDir;
in
{
  description = "GitHub Actions runner";

  wantedBy = [ "multi-user.target" ];
  wants = [ "network-online.target" ];
  after = [ "network.target" "network-online.target" ];

  environment = {
    HOME = workDir;
    RUNNER_ROOT = stateDir;
  } // cfg.extraEnvironment;

  path = (with pkgs; [
    bash
    coreutils
    git
    gnutar
    gzip
  ]) ++ [
    config.nix.package
  ] ++ cfg.extraPackages;

  serviceConfig = mkMerge [
    {
      ExecStart = "${cfg.package}/bin/Runner.Listener run --startuptype service";

      # Does the following, sequentially:
      # - If the module configuration or the token has changed, purge the state directory,
      #   and create the current and the new token file with the contents of the configured
      #   token. While both files have the same content, only the later is accessible by
      #   the service user.
      # - Configure the runner using the new token file. When finished, delete it.
      # - Set up the directory structure by creating the necessary symlinks.
      ExecStartPre =
        let
          # Wrapper script which expects the full path of the state, working and logs
          # directory as arguments. Overrides the respective systemd variables to provide
          # unambiguous directory names. This becomes relevant, for example, if the
          # caller overrides any of the StateDirectory=, RuntimeDirectory= or LogDirectory=
          # to contain more than one directory. This causes systemd to set the respective
          # environment variables with the path of all of the given directories, separated
          # by a colon.
          writeScript = name: lines: pkgs.writeShellScript "${svcName}-${name}.sh" ''
            set -euo pipefail

            STATE_DIRECTORY="$1"
            WORK_DIRECTORY="$2"
            LOGS_DIRECTORY="$3"

            ${lines}
          '';
          runnerRegistrationConfig = getAttrs [ "name" "tokenFile" "url" "runnerGroup" "extraLabels" "ephemeral" "workDir" ] cfg;
          newConfigPath = builtins.toFile "${svcName}-config.json" (builtins.toJSON runnerRegistrationConfig);
          currentConfigPath = "$STATE_DIRECTORY/.nixos-current-config.json";
          newConfigTokenPath = "$STATE_DIRECTORY/.new-token";
          currentConfigTokenPath = "$STATE_DIRECTORY/${currentConfigTokenFilename}";

          runnerCredFiles = [
            ".credentials"
            ".credentials_rsaparams"
            ".runner"
          ];
          unconfigureRunner = writeScript "unconfigure" ''
            copy_tokens() {
              # Copy the configured token file to the state dir and allow the service user to read the file
              install --mode=666 ${escapeShellArg cfg.tokenFile} "${newConfigTokenPath}"
              # Also copy current file to allow for a diff on the next start
              install --mode=600 ${escapeShellArg cfg.tokenFile} "${currentConfigTokenPath}"
            }
            clean_state() {
              find "$STATE_DIRECTORY/" -mindepth 1 -delete
              copy_tokens
            }
            diff_config() {
              changed=0
              # Check for module config changes
              [[ -f "${currentConfigPath}" ]] \
                && ${pkgs.diffutils}/bin/diff -q '${newConfigPath}' "${currentConfigPath}" >/dev/null 2>&1 \
                || changed=1
              # Also check the content of the token file
              [[ -f "${currentConfigTokenPath}" ]] \
                && ${pkgs.diffutils}/bin/diff -q "${currentConfigTokenPath}" ${escapeShellArg cfg.tokenFile} >/dev/null 2>&1 \
                || changed=1
              # If the config has changed, remove old state and copy tokens
              if [[ "$changed" -eq 1 ]]; then
                echo "Config has changed, removing old runner state."
                echo "The old runner will still appear in the GitHub Actions UI." \
                     "You have to remove it manually."
                clean_state
              fi
            }
            if [[ "${optionalString cfg.ephemeral "1"}" ]]; then
              # In ephemeral mode, we always want to start with a clean state
              clean_state
            elif [[ "$(ls -A "$STATE_DIRECTORY")" ]]; then
              # There are state files from a previous run; diff them to decide if we need a new registration
              diff_config
            else
              # The state directory is entirely empty which indicates a first start
              copy_tokens
            fi
            # Always clean workDir
            find -H "$WORK_DIRECTORY" -mindepth 1 -delete
          '';
          configureRunner = writeScript "configure" ''
            if [[ -e "${newConfigTokenPath}" ]]; then
              echo "Configuring GitHub Actions Runner"
              args=(
                --unattended
                --disableupdate
                --work "$WORK_DIRECTORY"
                --url ${escapeShellArg cfg.url}
                --labels ${escapeShellArg (concatStringsSep "," cfg.extraLabels)}
                --name ${escapeShellArg cfg.name}
                ${optionalString cfg.replace "--replace"}
                ${optionalString (cfg.runnerGroup != null) "--runnergroup ${escapeShellArg cfg.runnerGroup}"}
                ${optionalString cfg.ephemeral "--ephemeral"}
              )
              # If the token file contains a PAT (i.e., it starts with "ghp_" or "github_pat_"), we have to use the --pat option,
              # if it is not a PAT, we assume it contains a registration token and use the --token option
              token=$(<"${newConfigTokenPath}")
              if [[ "$token" =~ ^ghp_* ]] || [[ "$token" =~ ^github_pat_* ]]; then
                args+=(--pat "$token")
              else
                args+=(--token "$token")
              fi
              ${cfg.package}/bin/config.sh "''${args[@]}"
              # Move the automatically created _diag dir to the logs dir
              mkdir -p  "$STATE_DIRECTORY/_diag"
              cp    -r  "$STATE_DIRECTORY/_diag/." "$LOGS_DIRECTORY/"
              rm    -rf "$STATE_DIRECTORY/_diag/"
              # Cleanup token from config
              rm "${newConfigTokenPath}"
              # Symlink to new config
              ln -s '${newConfigPath}' "${currentConfigPath}"
            fi
          '';
          setupWorkDir = writeScript "setup-work-dirs" ''
            # Link _diag dir
            ln -s "$LOGS_DIRECTORY" "$WORK_DIRECTORY/_diag"

            # Link the runner credentials to the work dir
            ln -s "$STATE_DIRECTORY"/{${lib.concatStringsSep "," runnerCredFiles}} "$WORK_DIRECTORY/"
          '';
        in
        map (x: "${x} ${escapeShellArgs [ stateDir workDir logsDir ]}") [
          "+${unconfigureRunner}" # runs as root
          configureRunner
          setupWorkDir
        ];

      # If running in ephemeral mode, restart the service on-exit (i.e., successful de-registration of the runner)
      # to trigger a fresh registration.
      Restart = if cfg.ephemeral then "on-success" else "no";
      # If the runner exits with `ReturnCode.RetryableError = 2`, always restart the service:
      # https://github.com/actions/runner/blob/40ed7f8/src/Runner.Common/Constants.cs#L146
      RestartForceExitStatus = [ 2 ];

      # Contains _diag
      LogsDirectory = [ systemdDir ];
      # Default RUNNER_ROOT which contains ephemeral Runner data
      RuntimeDirectory = [ systemdDir ];
      # Home of persistent runner data, e.g., credentials
      StateDirectory = [ systemdDir ];
      StateDirectoryMode = "0700";
      WorkingDirectory = workDir;

      InaccessiblePaths = [
        # Token file path given in the configuration, if visible to the service
        "-${cfg.tokenFile}"
        # Token file in the state directory
        "${stateDir}/${currentConfigTokenFilename}"
      ];

      KillSignal = "SIGINT";

      # Hardening (may overlap with DynamicUser=)
      # The following options are only for optimizing:
      # systemd-analyze security github-runner
      AmbientCapabilities = mkBefore [ "" ];
      CapabilityBoundingSet = mkBefore [ "" ];
      # ProtectClock= adds DeviceAllow=char-rtc r
      DeviceAllow = mkBefore [ "" ];
      NoNewPrivileges = mkDefault true;
      PrivateDevices = mkDefault true;
      PrivateMounts = mkDefault true;
      PrivateTmp = mkDefault true;
      PrivateUsers = mkDefault true;
      ProtectClock = mkDefault true;
      ProtectControlGroups = mkDefault true;
      ProtectHome = mkDefault true;
      ProtectHostname = mkDefault true;
      ProtectKernelLogs = mkDefault true;
      ProtectKernelModules = mkDefault true;
      ProtectKernelTunables = mkDefault true;
      ProtectSystem = mkDefault "strict";
      RemoveIPC = mkDefault true;
      RestrictNamespaces = mkDefault true;
      RestrictRealtime = mkDefault true;
      RestrictSUIDSGID = mkDefault true;
      UMask = mkDefault "0066";
      ProtectProc = mkDefault "invisible";
      SystemCallFilter = mkBefore [
        "~@clock"
        "~@cpu-emulation"
        "~@module"
        "~@mount"
        "~@obsolete"
        "~@raw-io"
        "~@reboot"
        "~capset"
        "~setdomainname"
        "~sethostname"
      ];
      RestrictAddressFamilies = mkBefore [ "AF_INET" "AF_INET6" "AF_UNIX" "AF_NETLINK" ];

      BindPaths = lib.optionals (cfg.workDir != null) [ cfg.workDir ];

      # Needs network access
      PrivateNetwork = mkDefault false;
      # Cannot be true due to Node
      MemoryDenyWriteExecute = mkDefault false;

      # The more restrictive "pid" option makes `nix` commands in CI emit
      # "GC Warning: Couldn't read /proc/stat"
      # You may want to set this to "pid" if not using `nix` commands
      ProcSubset = mkDefault "all";
      # Coverage programs for compiled code such as `cargo-tarpaulin` disable
      # ASLR (address space layout randomization) which requires the
      # `personality` syscall
      # You may want to set this to `true` if not using coverage tooling on
      # compiled code
      LockPersonality = mkDefault false;

      # Note that this has some interactions with the User setting; so you may
      # want to consult the systemd docs if using both.
      DynamicUser = mkDefault true;
    }
    (mkIf (cfg.user != null) { User = cfg.user; })
    cfg.serviceOverrides
  ];
}