{ lib, pkgs, ... }:

with lib;

let

  makeScript =
    name: service:
    pkgs.writeScript "${name}-runner" ''
      #! ${pkgs.perl.withPackages (p: [ p.FileSlurp ])}/bin/perl -w

      use File::Slurp;

      sub run {
          my ($cmd) = @_;
          my @args = ();
          while ($cmd =~ /([^ \t\n']+)|(\'([^'])\')\s*/g) {
            push @args, $1;
          }
          my $prog;
          if (substr($args[0], 0, 1) eq "@") {
              $prog = substr($args[0], 1);
              shift @args;
          } else {
              $prog = $args[0];
          }
          my $pid = fork;
          if ($pid == 0) {
              setpgrp; # don't receive SIGINT etc. from terminal
              exec { $prog } @args;
              die "failed to exec $prog\n";
          } elsif (!defined $pid) {
              die "failed to fork: $!\n";
          }
          return $pid;
      };

      sub run_wait {
          my ($cmd) = @_;
          my $pid = run $cmd;
          die if waitpid($pid, 0) != $pid;
          return $?;
      };

      # Set the environment.  FIXME: escaping.
      foreach my $key (keys %ENV) {
          next if $key eq 'LOCALE_ARCHIVE';
          delete $ENV{$key};
      }
      ${concatStrings (
        mapAttrsToList (n: v: ''
          $ENV{'${n}'} = '${v}';
        '') service.environment
      )}

      # Run the ExecStartPre program.  FIXME: this could be a list.
      my $preStart = <<END_CMD;
      ${concatStringsSep "\n" (service.serviceConfig.ExecStartPre or [ ])}
      END_CMD
      if (defined $preStart && $preStart ne "\n") {
          print STDERR "running ExecStartPre: $preStart\n";
          my $res = run_wait $preStart;
          die "$0: ExecStartPre failed with status $res\n" if $res;
      };

      # Run the ExecStart program.
      my $cmd = <<END_CMD;
      ${service.serviceConfig.ExecStart}
      END_CMD

      print STDERR "running ExecStart: $cmd\n";
      my $mainPid = run $cmd;
      $ENV{'MAINPID'} = $mainPid;

      # Catch SIGINT, propagate to the main program.
      sub intHandler {
          print STDERR "got SIGINT, stopping service...\n";
          kill 'INT', $mainPid;
      };
      $SIG{'INT'} = \&intHandler;
      $SIG{'QUIT'} = \&intHandler;

      # Run the ExecStartPost program.
      my $postStart = <<END_CMD;
      ${concatStringsSep "\n" (service.serviceConfig.ExecStartPost or [ ])}
      END_CMD
      if (defined $postStart && $postStart ne "\n") {
          print STDERR "running ExecStartPost: $postStart\n";
          my $res = run_wait $postStart;
          die "$0: ExecStartPost failed with status $res\n" if $res;
      }

      # Wait for the main program to exit.
      die if waitpid($mainPid, 0) != $mainPid;
      my $mainRes = $?;

      # Run the ExecStopPost program.
      my $postStop = <<END_CMD;
      ${service.serviceConfig.ExecStopPost or ""}
      END_CMD
      if (defined $postStop && $postStop ne "\n") {
          print STDERR "running ExecStopPost: $postStop\n";
          my $res = run_wait $postStop;
          die "$0: ExecStopPost failed with status $res\n" if $res;
      }

      exit($mainRes & 127 ? 255 : $mainRes << 8);
    '';

  opts =
    { config, name, ... }:
    {
      options.runner = mkOption {
        internal = true;
        description = ''
          A script that runs the service outside of systemd,
          useful for testing or for using NixOS services outside
          of NixOS.
        '';
      };
      config.runner = makeScript name config;
    };

in

{
  options = {
    systemd.services = mkOption {
      type = with types; attrsOf (submodule opts);
    };
  };
}