{ 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"

          ${lines}
        '';
        runnerRegistrationConfig = getAttrs [ "name" "tokenFile" "url" "runnerGroup" "extraLabels" "ephemeral" ] cfg;
        newConfigPath = builtins.toFile "${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 "$STATE_DIRECTORY")" ]]; then
            # There are state files from a previous run; diff them to decide if we need a new registration
            diff_config
          else
            # The state directory is entirely empty which indicates a first start
            copy_tokens
          fi        '';
        configureRunner = writeScript "configure" ''
          if [[ -e "${newConfigTokenPath}" ]]; 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}"}
              ${optionalString cfg.ephemeral "--ephemeral"}
            )
            # If the token file contains a PAT (i.e., it starts with "ghp_" or "github_pat_"), we have to use the --pat option,
            # if it is not a PAT, we assume it contains a registration token and use the --token option
            token=$(<"${newConfigTokenPath}")
            if [[ "$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}' "${currentConfigPath}"
          fi
        '';
        setupRuntimeDir = writeScript "setup-runtime-dirs" ''
          # Link _diag dir
          ln -s "$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";
    # 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 = runtimeDir;

    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 = "";
    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;

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