{ config, lib, pkgs, utils, ... }:

with lib;
with utils;
with import ./systemd-unit-options.nix { inherit config lib; };

let

  cfg = config.systemd;

  systemd = cfg.package;

  makeUnit = name: unit:
    if unit.enable then
      pkgs.runCommand "unit" { preferLocalBuild = true; inherit (unit) text; }
        ''
          mkdir -p $out
          echo -n "$text" > $out/${name}
        ''
    else
      pkgs.runCommand "unit" { preferLocalBuild = true; }
        ''
          mkdir -p $out
          ln -s /dev/null $out/${name}
        '';

  upstreamSystemUnits =
    [ # Targets.
      "basic.target"
      "sysinit.target"
      "sockets.target"
      "graphical.target"
      "multi-user.target"
      "network.target"
      "network-online.target"
      "nss-lookup.target"
      "nss-user-lookup.target"
      "time-sync.target"
      #"cryptsetup.target"
      "sigpwr.target"
      "timers.target"
      "paths.target"
      "rpcbind.target"

      # Rescue mode.
      "rescue.target"
      "rescue.service"

      # Udev.
      "systemd-udevd-control.socket"
      "systemd-udevd-kernel.socket"
      "systemd-udevd.service"
      "systemd-udev-settle.service"
      "systemd-udev-trigger.service"

      # Consoles.
      "getty.target"
      "getty@.service"
      "serial-getty@.service"
      "container-getty@.service"
      "systemd-vconsole-setup.service"

      # Hardware (started by udev when a relevant device is plugged in).
      "sound.target"
      "bluetooth.target"
      "printer.target"
      "smartcard.target"

      # Login stuff.
      "systemd-logind.service"
      "autovt@.service"
      #"systemd-vconsole-setup.service"
      "systemd-user-sessions.service"
      "dbus-org.freedesktop.login1.service"
      "dbus-org.freedesktop.machine1.service"
      "user@.service"

      # Journal.
      "systemd-journald.socket"
      "systemd-journald.service"
      "systemd-journal-flush.service"
      "systemd-journal-gatewayd.socket"
      "systemd-journal-gatewayd.service"
      "syslog.socket"

      # SysV init compatibility.
      "systemd-initctl.socket"
      "systemd-initctl.service"

      # Kernel module loading.
      "systemd-modules-load.service"
      "kmod-static-nodes.service"

      # Filesystems.
      "systemd-fsck@.service"
      "systemd-fsck-root.service"
      "systemd-remount-fs.service"
      "local-fs.target"
      "local-fs-pre.target"
      "remote-fs.target"
      "remote-fs-pre.target"
      "swap.target"
      "dev-hugepages.mount"
      "dev-mqueue.mount"
      "proc-sys-fs-binfmt_misc.mount"
      "sys-fs-fuse-connections.mount"
      "sys-kernel-config.mount"
      "sys-kernel-debug.mount"

      # Maintaining state across reboots.
      "systemd-random-seed.service"

      # Hibernate / suspend.
      "hibernate.target"
      "suspend.target"
      "sleep.target"
      "hybrid-sleep.target"
      "systemd-hibernate.service"
      "systemd-suspend.service"
      "systemd-hybrid-sleep.service"
      "systemd-shutdownd.socket"
      "systemd-shutdownd.service"

      # Reboot stuff.
      "reboot.target"
      "systemd-reboot.service"
      "poweroff.target"
      "systemd-poweroff.service"
      "halt.target"
      "systemd-halt.service"
      "ctrl-alt-del.target"
      "shutdown.target"
      "umount.target"
      "final.target"
      "kexec.target"
      "systemd-kexec.service"
      "systemd-update-utmp.service"

      # Password entry.
      "systemd-ask-password-console.path"
      "systemd-ask-password-console.service"
      "systemd-ask-password-wall.path"
      "systemd-ask-password-wall.service"

      # Slices / containers.
      "slices.target"
      "-.slice"
      "system.slice"
      "user.slice"
      "machine.slice"
      "systemd-machined.service"

      # Temporary file creation / cleanup.
      "systemd-tmpfiles-clean.service"
      "systemd-tmpfiles-clean.timer"
      "systemd-tmpfiles-setup.service"
      "systemd-tmpfiles-setup-dev.service"

      # Misc.
      "systemd-sysctl.service"
    ]

    ++ optionals cfg.enableEmergencyMode [
      "emergency.target"
      "emergency.service"
    ];

  upstreamSystemWants =
    [ #"basic.target.wants"
      "sysinit.target.wants"
      "sockets.target.wants"
      "local-fs.target.wants"
      "multi-user.target.wants"
      "timers.target.wants"
    ];

  upstreamUserUnits =
    [ "basic.target"
      "default.target"
      "exit.target"
      "paths.target"
      "shutdown.target"
      "sockets.target"
      "systemd-exit.service"
      "timers.target"
    ];

  makeJobScript = name: text:
    let x = pkgs.writeTextFile { name = "unit-script"; executable = true; destination = "/bin/${name}"; inherit text; };
    in "${x}/bin/${name}";

  unitConfig = { name, config, ... }: {
    config = {
      unitConfig =
        optionalAttrs (config.requires != [])
          { Requires = toString config.requires; }
        // optionalAttrs (config.wants != [])
          { Wants = toString config.wants; }
        // optionalAttrs (config.after != [])
          { After = toString config.after; }
        // optionalAttrs (config.before != [])
          { Before = toString config.before; }
        // optionalAttrs (config.bindsTo != [])
          { BindsTo = toString config.bindsTo; }
        // optionalAttrs (config.partOf != [])
          { PartOf = toString config.partOf; }
        // optionalAttrs (config.conflicts != [])
          { Conflicts = toString config.conflicts; }
        // optionalAttrs (config.restartTriggers != [])
          { X-Restart-Triggers = toString config.restartTriggers; }
        // optionalAttrs (config.description != "") {
          Description = config.description;
        };
    };
  };

  serviceConfig = { name, config, ... }: {
    config = mkMerge
      [ { # Default path for systemd services.  Should be quite minimal.
          path =
            [ pkgs.coreutils
              pkgs.findutils
              pkgs.gnugrep
              pkgs.gnused
              systemd
            ];
          environment.PATH = config.path;
        }
        (mkIf (config.preStart != "")
          { serviceConfig.ExecStartPre = makeJobScript "${name}-pre-start" ''
              #! ${pkgs.stdenv.shell} -e
              ${config.preStart}
            '';
          })
        (mkIf (config.script != "")
          { serviceConfig.ExecStart = makeJobScript "${name}-start" ''
              #! ${pkgs.stdenv.shell} -e
              ${config.script}
            '' + " " + config.scriptArgs;
          })
        (mkIf (config.postStart != "")
          { serviceConfig.ExecStartPost = makeJobScript "${name}-post-start" ''
              #! ${pkgs.stdenv.shell} -e
              ${config.postStart}
            '';
          })
        (mkIf (config.preStop != "")
          { serviceConfig.ExecStop = makeJobScript "${name}-pre-stop" ''
              #! ${pkgs.stdenv.shell} -e
              ${config.preStop}
            '';
          })
        (mkIf (config.postStop != "")
          { serviceConfig.ExecStopPost = makeJobScript "${name}-post-stop" ''
              #! ${pkgs.stdenv.shell} -e
              ${config.postStop}
            '';
          })
      ];
  };

  mountConfig = { name, config, ... }: {
    config = {
      mountConfig =
        { What = config.what;
          Where = config.where;
        } // optionalAttrs (config.type != "") {
          Type = config.type;
        } // optionalAttrs (config.options != "") {
          Options = config.options;
        };
    };
  };

  automountConfig = { name, config, ... }: {
    config = {
      automountConfig =
        { Where = config.where;
        };
    };
  };

  toOption = x:
    if x == true then "true"
    else if x == false then "false"
    else toString x;

  attrsToSection = as:
    concatStrings (concatLists (mapAttrsToList (name: value:
      map (x: ''
          ${name}=${toOption x}
        '')
        (if isList value then value else [value]))
        as));

  commonUnitText = def: ''
      [Unit]
      ${attrsToSection def.unitConfig}
    '';

  targetToUnit = name: def:
    { inherit (def) wantedBy requiredBy enable;
      text =
        ''
          [Unit]
          ${attrsToSection def.unitConfig}
        '';
    };

  serviceToUnit = name: def:
    { inherit (def) wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Service]
          ${let env = cfg.globalEnvironment // def.environment;
            in concatMapStrings (n: "Environment=\"${n}=${getAttr n env}\"\n") (attrNames env)}
          ${if def.reloadIfChanged then ''
            X-ReloadIfChanged=true
          '' else if !def.restartIfChanged then ''
            X-RestartIfChanged=false
          '' else ""}
          ${optionalString (!def.stopIfChanged) "X-StopIfChanged=false"}
          ${attrsToSection def.serviceConfig}
        '';
    };

  socketToUnit = name: def:
    { inherit (def) wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Socket]
          ${attrsToSection def.socketConfig}
          ${concatStringsSep "\n" (map (s: "ListenStream=${s}") def.listenStreams)}
        '';
    };

  timerToUnit = name: def:
    { inherit (def) wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Timer]
          ${attrsToSection def.timerConfig}
        '';
    };

  pathToUnit = name: def:
    { inherit (def) wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Path]
          ${attrsToSection def.pathConfig}
        '';
    };

  mountToUnit = name: def:
    { inherit (def) wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Mount]
          ${attrsToSection def.mountConfig}
        '';
    };

  automountToUnit = name: def:
    { inherit (def) wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Automount]
          ${attrsToSection def.automountConfig}
        '';
    };

  generateUnits = type: units: upstreamUnits: upstreamWants:
    pkgs.runCommand "${type}-units" { preferLocalBuild = true; } ''
      mkdir -p $out

      # Copy the upstream systemd units we're interested in.
      for i in ${toString upstreamUnits}; do
        fn=${systemd}/example/systemd/${type}/$i
        if ! [ -e $fn ]; then echo "missing $fn"; false; fi
        if [ -L $fn ]; then
          target="$(readlink "$fn")"
          if [ ''${target:0:3} = ../ ]; then
            ln -s "$(readlink -f "$fn")" $out/
          else
            cp -pd $fn $out/
          fi
        else
          ln -s $fn $out/
        fi
      done

      # Copy .wants links, but only those that point to units that
      # we're interested in.
      for i in ${toString upstreamWants}; do
        fn=${systemd}/example/systemd/${type}/$i
        if ! [ -e $fn ]; then echo "missing $fn"; false; fi
        x=$out/$(basename $fn)
        mkdir $x
        for i in $fn/*; do
          y=$x/$(basename $i)
          cp -pd $i $y
          if ! [ -e $y ]; then rm $y; fi
        done
      done

      # Symlink all units provided listed in systemd.packages.
      for i in ${toString cfg.packages}; do
        files=$(echo $i/etc/systemd/${type}/* $i/lib/systemd/${type}/*)
        if [ -n "$files" ]; then
          ln -s $files $out/
        fi
      done

      # Symlink all units defined by systemd.units. If these are also
      # provided by systemd or systemd.packages, then add them as
      # <unit-name>.d/overrides.conf, which makes them extend the
      # upstream unit.
      for i in ${toString (mapAttrsToList (n: v: v.unit) units)}; do
        fn=$(basename $i/*)
        if [ -e $out/$fn ]; then
          if [ "$(readlink -f $i/$fn)" = /dev/null ]; then
            ln -sfn /dev/null $out/$fn
          else
            mkdir $out/$fn.d
            ln -s $i/$fn $out/$fn.d/overrides.conf
          fi
       else
          ln -fs $i/$fn $out/
        fi
      done

      # Created .wants and .requires symlinks from the wantedBy and
      # requiredBy options.
      ${concatStrings (mapAttrsToList (name: unit:
          concatMapStrings (name2: ''
            mkdir -p $out/'${name2}.wants'
            ln -sfn '../${name}' $out/'${name2}.wants'/
          '') unit.wantedBy) units)}

      ${concatStrings (mapAttrsToList (name: unit:
          concatMapStrings (name2: ''
            mkdir -p $out/'${name2}.requires'
            ln -sfn '../${name}' $out/'${name2}.requires'/
          '') unit.requiredBy) units)}

      ${optionalString (type == "system") ''
        # Stupid misc. symlinks.
        ln -s ${cfg.defaultUnit} $out/default.target

        ln -s rescue.target $out/kbrequest.target

        mkdir -p $out/getty.target.wants/
        ln -s ../autovt@tty1.service $out/getty.target.wants/

        ln -s ../local-fs.target ../remote-fs.target ../network.target ../nss-lookup.target \
              ../nss-user-lookup.target ../swap.target $out/multi-user.target.wants/
      ''}
    ''; # */

in

{

  ###### interface

  options = {

    systemd.package = mkOption {
      default = pkgs.systemd;
      type = types.package;
      description = "The systemd package.";
    };

    systemd.units = mkOption {
      description = "Definition of systemd units.";
      default = {};
      type = types.attrsOf types.optionSet;
      options = { name, config, ... }:
        { options = concreteUnitOptions;
          config = {
            unit = mkDefault (makeUnit name config);
          };
        };
    };

    systemd.packages = mkOption {
      default = [];
      type = types.listOf types.package;
      description = "Packages providing systemd units.";
    };

    systemd.targets = mkOption {
      default = {};
      type = types.attrsOf types.optionSet;
      options = [ targetOptions unitConfig ];
      description = "Definition of systemd target units.";
    };

    systemd.services = mkOption {
      default = {};
      type = types.attrsOf types.optionSet;
      options = [ serviceOptions unitConfig serviceConfig ];
      description = "Definition of systemd service units.";
    };

    systemd.sockets = mkOption {
      default = {};
      type = types.attrsOf types.optionSet;
      options = [ socketOptions unitConfig ];
      description = "Definition of systemd socket units.";
    };

    systemd.timers = mkOption {
      default = {};
      type = types.attrsOf types.optionSet;
      options = [ timerOptions unitConfig ];
      description = "Definition of systemd timer units.";
    };

    systemd.paths = mkOption {
      default = {};
      type = types.attrsOf types.optionSet;
      options = [ pathOptions unitConfig ];
      description = "Definition of systemd path units.";
    };

    systemd.mounts = mkOption {
      default = [];
      type = types.listOf types.optionSet;
      options = [ mountOptions unitConfig mountConfig ];
      description = ''
        Definition of systemd mount units.
        This is a list instead of an attrSet, because systemd mandates the names to be derived from
        the 'where' attribute.
      '';
    };

    systemd.automounts = mkOption {
      default = [];
      type = types.listOf types.optionSet;
      options = [ automountOptions unitConfig automountConfig ];
      description = ''
        Definition of systemd automount units.
        This is a list instead of an attrSet, because systemd mandates the names to be derived from
        the 'where' attribute.
      '';
    };

    systemd.defaultUnit = mkOption {
      default = "multi-user.target";
      type = types.str;
      description = "Default unit started when the system boots.";
    };

    systemd.globalEnvironment = mkOption {
      type = types.attrs;
      default = {};
      example = { TZ = "CET"; };
      description = ''
        Environment variables passed to <emphasis>all</emphasis> systemd units.
      '';
    };

    systemd.extraConfig = mkOption {
      default = "";
      type = types.lines;
      example = "DefaultLimitCORE=infinity";
      description = ''
        Extra config options for systemd. See man systemd-system.conf for
        available options.
      '';
    };

    services.journald.console = mkOption {
      default = "";
      type = types.str;
      description = "If non-empty, write log messages to the specified TTY device.";
    };

    services.journald.rateLimitInterval = mkOption {
      default = "10s";
      type = types.str;
      description = ''
        Configures the rate limiting interval that is applied to all
        messages generated on the system. This rate limiting is applied
        per-service, so that two services which log do not interfere with
        each other's limit. The value may be specified in the following
        units: s, min, h, ms, us. To turn off any kind of rate limiting,
        set either value to 0.
      '';
    };

    services.journald.rateLimitBurst = mkOption {
      default = 100;
      type = types.uniq types.int;
      description = ''
        Configures the rate limiting burst limit (number of messages per
        interval) that is applied to all messages generated on the system.
        This rate limiting is applied per-service, so that two services
        which log do not interfere with each other's limit.
      '';
    };

    services.journald.extraConfig = mkOption {
      default = "";
      type = types.lines;
      example = "Storage=volatile";
      description = ''
        Extra config options for systemd-journald. See man journald.conf
        for available options.
      '';
    };

    services.journald.enableHttpGateway = mkOption {
      default = false;
      type = types.bool;
      description = ''
        Whether to enable the HTTP gateway to the journal.
      '';
    };

    services.logind.extraConfig = mkOption {
      default = "";
      type = types.lines;
      example = "HandleLidSwitch=ignore";
      description = ''
        Extra config options for systemd-logind. See man logind.conf for
        available options.
      '';
    };

    systemd.enableEmergencyMode = mkOption {
      default = true;
      type = types.bool;
      description = ''
        Whether to enable emergency mode, which is an
        <command>sulogin</command> shell started on the console if
        mounting a filesystem fails.  Since some machines (like EC2
        instances) have no console of any kind, emergency mode doesn't
        make sense, and it's better to continue with the boot insofar
        as possible.
      '';
    };

    systemd.tmpfiles.rules = mkOption {
      type = types.listOf types.str;
      default = [];
      example = [ "d /tmp 1777 root root 10d" ];
      description = ''
        Rules for creating and cleaning up temporary files
        automatically. See
        <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for the exact format. You should not use this option to create
        files required by systemd services, since there is no
        guarantee that <command>systemd-tmpfiles</command> runs when
        the system is reconfigured using
        <command>nixos-rebuild</command>.
      '';
    };

    systemd.user.units = mkOption {
      description = "Definition of systemd per-user units.";
      default = {};
      type = types.attrsOf types.optionSet;
      options = { name, config, ... }:
        { options = concreteUnitOptions;
          config = {
            unit = mkDefault (makeUnit name config);
          };
        };
    };

    systemd.user.services = mkOption {
      default = {};
      type = types.attrsOf types.optionSet;
      options = [ serviceOptions unitConfig serviceConfig ];
      description = "Definition of systemd per-user service units.";
    };

  };


  ###### implementation

  config = {

    assertions = mapAttrsToList (name: service: {
      assertion = service.serviceConfig.Type or "" == "oneshot" ->
                    builtins.elem (service.serviceConfig.Restart or "no") ["no" "on-failure" "on-abort"];
      message = "${name}: Type=oneshot services must have Restart=no";
    }) cfg.services;

    system.build.units = cfg.units;

    environment.systemPackages = [ systemd ];

    environment.etc."systemd/system".source =
      generateUnits "system" cfg.units upstreamSystemUnits upstreamSystemWants;

    environment.etc."systemd/user".source =
      generateUnits "user" cfg.user.units upstreamUserUnits [];

    environment.etc."systemd/system.conf".text =
      ''
        [Manager]
        ${config.systemd.extraConfig}
      '';

    environment.etc."systemd/journald.conf".text =
      ''
        [Journal]
        RateLimitInterval=${config.services.journald.rateLimitInterval}
        RateLimitBurst=${toString config.services.journald.rateLimitBurst}
        ${optionalString (config.services.journald.console != "") ''
          ForwardToConsole=yes
          TTYPath=${config.services.journald.console}
        ''}
        ${config.services.journald.extraConfig}
      '';

    environment.etc."systemd/logind.conf".text =
      ''
        [Login]
        ${config.services.logind.extraConfig}
      '';

    environment.etc."systemd/sleep.conf".text =
      ''
        [Sleep]
      '';

    system.activationScripts.systemd = stringAfter [ "groups" ]
      ''
        mkdir -m 0755 -p /var/lib/udev
        mkdir -p /var/log/journal
        chmod 0755 /var/log/journal

        # Make all journals readable to users in the wheel and adm
        # groups, in addition to those in the systemd-journal group.
        # Users can always read their own journals.
        ${pkgs.acl}/bin/setfacl -nm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal
      '';

    # Target for ‘charon send-keys’ to hook into.
    users.extraGroups.keys.gid = config.ids.gids.keys;

    systemd.targets.keys =
      { description = "Security Keys";
        unitConfig.X-StopOnReconfiguration = true;
      };

    systemd.units =
      mapAttrs' (n: v: nameValuePair "${n}.target" (targetToUnit n v)) cfg.targets
      // mapAttrs' (n: v: nameValuePair "${n}.service" (serviceToUnit n v)) cfg.services
      // mapAttrs' (n: v: nameValuePair "${n}.socket" (socketToUnit n v)) cfg.sockets
      // mapAttrs' (n: v: nameValuePair "${n}.timer" (timerToUnit n v)) cfg.timers
      // mapAttrs' (n: v: nameValuePair "${n}.path" (pathToUnit n v)) cfg.paths
      // listToAttrs (map
                   (v: let n = escapeSystemdPath v.where;
                       in nameValuePair "${n}.mount" (mountToUnit n v)) cfg.mounts)
      // listToAttrs (map
                   (v: let n = escapeSystemdPath v.where;
                       in nameValuePair "${n}.automount" (automountToUnit n v)) cfg.automounts);

    systemd.user.units =
      mapAttrs' (n: v: nameValuePair "${n}.service" (serviceToUnit n v)) cfg.user.services;

    system.requiredKernelConfig = map config.lib.kernelConfig.isEnabled [
      "CGROUPS" "AUTOFS4_FS" "DEVTMPFS"
    ];

    environment.shellAliases =
      { start = "systemctl start";
        stop = "systemctl stop";
        restart = "systemctl restart";
        status = "systemctl status";
      };

    users.extraGroups.systemd-journal.gid = config.ids.gids.systemd-journal;
    users.extraUsers.systemd-journal-gateway.uid = config.ids.uids.systemd-journal-gateway;
    users.extraGroups.systemd-journal-gateway.gid = config.ids.gids.systemd-journal-gateway;

    # Generate timer units for all services that have a ‘startAt’ value.
    systemd.timers =
      mapAttrs (name: service:
        { wantedBy = [ "timers.target" ];
          timerConfig.OnCalendar = service.startAt;
        })
        (filterAttrs (name: service: service.startAt != "") cfg.services);

    systemd.sockets.systemd-journal-gatewayd.wantedBy =
      optional config.services.journald.enableHttpGateway "sockets.target";

    # Provide the systemd-user PAM service, required to run systemd
    # user instances.
    security.pam.services.systemd-user =
      { # Ensure that pam_systemd gets included. This is special-cased
        # in systemd to provide XDG_RUNTIME_DIR.
        startSession = true;
      };

    environment.etc."tmpfiles.d/x11.conf".source = "${systemd}/example/tmpfiles.d/x11.conf";

    environment.etc."tmpfiles.d/nixos.conf".text =
      ''
        # This file is created automatically and should not be modified.
        # Please change the option ‘systemd.tmpfiles.rules’ instead.
        ${concatStringsSep "\n" cfg.tmpfiles.rules}
      '';

  };
}