{ 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;

{
  description = "GitHub Actions runner";

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

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

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

  serviceConfig = rec {
    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, runtime 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"
          RUNTIME_DIRECTORY="$2"
          LOGS_DIRECTORY="$3"

          ${lines}
        '';
        currentConfigPath = "$STATE_DIRECTORY/.nixos-current-config.json";
        runnerRegistrationConfig = getAttrs [ "name" "tokenFile" "url" "runnerGroup" "extraLabels" "ephemeral" ] cfg;
        newConfigPath = builtins.toFile "${svcName}-config.json" (builtins.toJSON runnerRegistrationConfig);
        newConfigTokenFilename = ".new-token";
        runnerCredFiles = [
          ".credentials"
          ".credentials_rsaparams"
          ".runner"
        ];
        unconfigureRunner = writeScript "unconfigure" ''
          differs=

          if [[ "$(ls -A "$STATE_DIRECTORY")" ]]; then
            # State directory is not empty
            # Set `differs = 1` if current and new runner config differ or if `currentConfigPath` does not exist
            ${pkgs.diffutils}/bin/diff -q '${newConfigPath}' "${currentConfigPath}" >/dev/null 2>&1 || differs=1
            # Also trigger a registration if the token content changed
            ${pkgs.diffutils}/bin/diff -q \
              "$STATE_DIRECTORY"/${currentConfigTokenFilename} \
              ${escapeShellArg cfg.tokenFile} \
              >/dev/null 2>&1 || differs=1
            # If .credentials does not exist, assume a previous run de-registered the runner on stop (ephemeral mode)
            [[ ! -f "$STATE_DIRECTORY/.credentials" ]] && differs=1
          fi

          if [[ -n "$differs" ]]; then
            echo "Config has changed, removing old runner state."
            # In ephemeral mode, the runner deletes the `.credentials` file after de-registering it with GitHub
            [[ -f "$STATE_DIRECTORY/.credentials" ]] && echo "The old runner will still appear in the GitHub Actions UI." \
              "You have to remove it manually."
            find "$STATE_DIRECTORY/" -mindepth 1 -delete

            # Copy the configured token file to the state dir and allow the service user to read the file
            install --mode=666 ${escapeShellArg cfg.tokenFile} "$STATE_DIRECTORY/${newConfigTokenFilename}"
            # Also copy current file to allow for a diff on the next start
            install --mode=600 ${escapeShellArg cfg.tokenFile} "$STATE_DIRECTORY/${currentConfigTokenFilename}"
          fi
        '';
        configureRunner = writeScript "configure" ''
          if [[ -e "$STATE_DIRECTORY/${newConfigTokenFilename}" ]]; then
            echo "Configuring GitHub Actions Runner"

            args=(
              --unattended
              --disableupdate
              --work "$RUNTIME_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_"), 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=$(<"$STATE_DIRECTORY/${newConfigTokenFilename}")
            if [[ "$token" =~ ^ghp_* ]]; 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 "$STATE_DIRECTORY/${newConfigTokenFilename}"

            # Symlink to new config
            ln -s '${newConfigPath}' "${currentConfigPath}"
          fi
        '';
        setupRuntimeDir = writeScript "setup-runtime-dirs" ''
          # Link _diag dir
          ln -s "$LOGS_DIRECTORY" "$RUNTIME_DIRECTORY/_diag"

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

    # 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";

    # 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 = runtimeDir;

    InaccessiblePaths = [
      # Token file path given in the configuration
      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 = "";
    CapabilityBoundingSet = "";
    # ProtectClock= adds DeviceAllow=char-rtc r
    DeviceAllow = "";
    NoNewPrivileges = true;
    PrivateDevices = true;
    PrivateMounts = true;
    PrivateTmp = true;
    PrivateUsers = true;
    ProtectClock = true;
    ProtectControlGroups = true;
    ProtectHome = true;
    ProtectHostname = true;
    ProtectKernelLogs = true;
    ProtectKernelModules = true;
    ProtectKernelTunables = true;
    ProtectSystem = "strict";
    RemoveIPC = true;
    RestrictNamespaces = true;
    RestrictRealtime = true;
    RestrictSUIDSGID = true;
    UMask = "0066";
    ProtectProc = "invisible";
    SystemCallFilter = [
      "~@clock"
      "~@cpu-emulation"
      "~@module"
      "~@mount"
      "~@obsolete"
      "~@raw-io"
      "~@reboot"
      "~capset"
      "~setdomainname"
      "~sethostname"
    ];
    RestrictAddressFamilies = [ "AF_INET" "AF_INET6" "AF_UNIX" "AF_NETLINK" ];

    # Needs network access
    PrivateNetwork = false;
    # Cannot be true due to Node
    MemoryDenyWriteExecute = 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 = "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 = false;
  } // (
    if cfg.user == null then { DynamicUser = true; } else { User = cfg.user; }
  ) // cfg.serviceOverrides;
}