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

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

let

  cfg = config.systemd;

  systemd = cfg.package;

  upstreamSystemUnits =
    [ # Targets.
      "basic.target"
      "sysinit.target"
      "sockets.target"
      "exit.target"
      "graphical.target"
      "multi-user.target"
      "network.target"
      "network-pre.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"
      # hwdb.bin is managed by NixOS
      # "systemd-hwdb-update.service"

      # Consoles.
      "getty.target"
      "getty-pre.target"
      "getty@.service"
      "serial-getty@.service"
      "console-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-user-sessions.service"
      "dbus-org.freedesktop.machine1.service"
      "user@.service"
      "user-runtime-dir@.service"

      # Journal.
      "systemd-journald.socket"
      "systemd-journald.service"
      "systemd-journal-flush.service"
      "systemd-journal-catalog-update.service"
      "systemd-journald-audit.socket"
      "systemd-journald-dev-log.socket"
      "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"
      "sys-fs-fuse-connections.mount"
      "sys-kernel-config.mount"
      "sys-kernel-debug.mount"

      # Maintaining state across reboots.
      "systemd-random-seed.service"
      "systemd-backlight@.service"
      "systemd-rfkill.service"
      "systemd-rfkill.socket"

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

      # Reboot stuff.
      "reboot.target"
      "systemd-reboot.service"
      "poweroff.target"
      "systemd-poweroff.service"
      "halt.target"
      "systemd-halt.service"
      "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"
      "user.slice"
      "machine.slice"
      "machines.target"
      "systemd-machined.service"
      "systemd-nspawn@.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"
      "dbus-org.freedesktop.timedate1.service"
      "dbus-org.freedesktop.locale1.service"
      "dbus-org.freedesktop.hostname1.service"
      "systemd-timedated.service"
      "systemd-localed.service"
      "systemd-hostnamed.service"
      "systemd-binfmt.service"
      "systemd-exit.service"
      "systemd-update-done.service"
    ] ++ optionals config.services.journald.enableHttpGateway [
      "systemd-journal-gatewayd.socket"
      "systemd-journal-gatewayd.service"
    ] ++ cfg.additionalUpstreamSystemUnits;

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

  upstreamUserUnits =
    [ "basic.target"
      "bluetooth.target"
      "default.target"
      "exit.target"
      "graphical-session-pre.target"
      "graphical-session.target"
      "paths.target"
      "printer.target"
      "shutdown.target"
      "smartcard.target"
      "sockets.target"
      "sound.target"
      "systemd-exit.service"
      "timers.target"
    ];

  makeJobScript = name: text:
    let mkScriptName =  s: "unit-script-" + (replaceChars [ "\\" "@" ] [ "-" "_" ] (shellEscape s) );
    in  pkgs.writeTextFile { name = mkScriptName name; executable = true; inherit text; };

  unitConfig = { config, options, ... }: {
    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.requisite != [])
          { Requisite = toString config.requisite; }
        // optionalAttrs (config.restartTriggers != [])
          { X-Restart-Triggers = toString config.restartTriggers; }
        // optionalAttrs (config.description != "") {
          Description = config.description; }
        // optionalAttrs (config.documentation != []) {
          Documentation = toString config.documentation; }
        // optionalAttrs (config.onFailure != []) {
          OnFailure = toString config.onFailure; }
        // optionalAttrs (options.startLimitIntervalSec.isDefined) {
          StartLimitIntervalSec = toString config.startLimitIntervalSec;
        };
    };
  };

  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.runtimeShell} -e
              ${config.preStart}
            '';
          })
        (mkIf (config.script != "")
          { serviceConfig.ExecStart = makeJobScript "${name}-start" ''
              #! ${pkgs.runtimeShell} -e
              ${config.script}
            '' + " " + config.scriptArgs;
          })
        (mkIf (config.postStart != "")
          { serviceConfig.ExecStartPost = makeJobScript "${name}-post-start" ''
              #! ${pkgs.runtimeShell} -e
              ${config.postStart}
            '';
          })
        (mkIf (config.reload != "")
          { serviceConfig.ExecReload = makeJobScript "${name}-reload" ''
              #! ${pkgs.runtimeShell} -e
              ${config.reload}
            '';
          })
        (mkIf (config.preStop != "")
          { serviceConfig.ExecStop = makeJobScript "${name}-pre-stop" ''
              #! ${pkgs.runtimeShell} -e
              ${config.preStop}
            '';
          })
        (mkIf (config.postStop != "")
          { serviceConfig.ExecStopPost = makeJobScript "${name}-post-stop" ''
              #! ${pkgs.runtimeShell} -e
              ${config.postStop}
            '';
          })
      ];
  };

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

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

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

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

  serviceToUnit = name: def:
    { inherit (def) aliases wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Service]
          ${let env = cfg.globalEnvironment // def.environment;
            in concatMapStrings (n:
              let s = optionalString (env."${n}" != null)
                "Environment=${builtins.toJSON "${n}=${env.${n}}"}\n";
              # systemd max line length is now 1MiB
              # https://github.com/systemd/systemd/commit/e6dde451a51dc5aaa7f4d98d39b8fe735f73d2af
              in if stringLength s >= 1048576 then throw "The value of the environment variable ‘${n}’ in systemd service ‘${name}.service’ is too long." else s) (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) aliases wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Socket]
          ${attrsToSection def.socketConfig}
          ${concatStringsSep "\n" (map (s: "ListenStream=${s}") def.listenStreams)}
        '';
    };

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

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

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

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

  sliceToUnit = name: def:
    { inherit (def) aliases wantedBy requiredBy enable;
      text = commonUnitText def +
        ''
          [Slice]
          ${attrsToSection def.sliceConfig}
        '';
    };

  logindHandlerType = types.enum [
    "ignore" "poweroff" "reboot" "halt" "kexec" "suspend"
    "hibernate" "hybrid-sleep" "suspend-then-hibernate" "lock"
  ];

in

{

  ###### interface

  options = {

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

    systemd.units = mkOption {
      description = "Definition of systemd units.";
      default = {};
      type = with types; attrsOf (submodule (
        { 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 = with types; attrsOf (submodule [ { options = targetOptions; } unitConfig] );
      description = "Definition of systemd target units.";
    };

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

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

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

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

    systemd.mounts = mkOption {
      default = [];
      type = with types; listOf (submodule [ { 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 = with types; listOf (submodule [ { 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.slices = mkOption {
      default = {};
      type = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig] );
      description = "Definition of slice configurations.";
    };

    systemd.generators = mkOption {
      type = types.attrsOf types.path;
      default = {};
      example = { "systemd-gpt-auto-generator" = "/dev/null"; };
      description = ''
        Definition of systemd generators.
        For each <literal>NAME = VALUE</literal> pair of the attrSet, a link is generated from
        <literal>/etc/systemd/system-generators/NAME</literal> to <literal>VALUE</literal>.
      '';
    };

    systemd.generator-packages = mkOption {
      default = [];
      type = types.listOf types.package;
      example = literalExample "[ pkgs.systemd-cryptsetup-generator ]";
      description = "Packages providing systemd generators.";
    };

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

    systemd.ctrlAltDelUnit = mkOption {
      default = "reboot.target";
      type = types.str;
      example = "poweroff.target";
      description = ''
        Target that should be started when Ctrl-Alt-Delete is pressed.
      '';
    };

    systemd.globalEnvironment = mkOption {
      type = with types; attrsOf (nullOr (either str (either path package)));
      default = {};
      example = { TZ = "CET"; };
      description = ''
        Environment variables passed to <emphasis>all</emphasis> systemd units.
      '';
    };

    systemd.enableCgroupAccounting = mkOption {
      default = false;
      type = types.bool;
      description = ''
        Whether to enable cgroup accounting.
      '';
    };

    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 = "30s";
      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 = 1000;
      type = 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.journald.forwardToSyslog = mkOption {
      default = config.services.rsyslogd.enable || config.services.syslog-ng.enable;
      defaultText = "services.rsyslogd.enable || services.syslog-ng.enable";
      type = types.bool;
      description = ''
        Whether to forward log messages to syslog.
      '';
    };

    services.logind.extraConfig = mkOption {
      default = "";
      type = types.lines;
      example = "IdleAction=lock";
      description = ''
        Extra config options for systemd-logind. See
        <link xlink:href="https://www.freedesktop.org/software/systemd/man/logind.conf.html">
        logind.conf(5)</link> for available options.
      '';
    };

    services.logind.killUserProcesses = mkOption {
      default = false;
      type = types.bool;
      description = ''
        Specifies whether the processes of a user should be killed
        when the user logs out.  If true, the scope unit corresponding
        to the session and all processes inside that scope will be
        terminated.  If false, the scope is "abandoned" (see
        <link xlink:href="https://www.freedesktop.org/software/systemd/man/systemd.scope.html#">
        systemd.scope(5)</link>), and processes are not killed.
        </para>

        <para>
        See <link xlink:href="https://www.freedesktop.org/software/systemd/man/logind.conf.html#KillUserProcesses=">logind.conf(5)</link>
        for more details.
      '';
    };

    services.logind.lidSwitch = mkOption {
      default = "suspend";
      example = "ignore";
      type = logindHandlerType;

      description = ''
        Specifies what to be done when the laptop lid is closed.
      '';
    };

    services.logind.lidSwitchDocked = mkOption {
      default = "ignore";
      example = "suspend";
      type = logindHandlerType;

      description = ''
        Specifies what to be done when the laptop lid is closed
        and another screen is added.
      '';
    };

    services.logind.lidSwitchExternalPower = mkOption {
      default = config.services.logind.lidSwitch;
      defaultText = "services.logind.lidSwitch";
      example = "ignore";
      type = logindHandlerType;

      description = ''
        Specifies what to do when the laptop lid is closed and the system is
        on external power. By default use the same action as specified in
        services.logind.lidSwitch.
      '';
    };

    systemd.user.extraConfig = mkOption {
      default = "";
      type = types.lines;
      example = "DefaultCPUAccounting=yes";
      description = ''
        Extra config options for systemd user instances. See man systemd-user.conf for
        available options.
      '';
    };

    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.
      '';
    };

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

    systemd.user.paths = mkOption {
      default = {};
      type = with types; attrsOf (submodule [ { options = pathOptions; } unitConfig ]);
      description = "Definition of systemd per-user path units.";
    };

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

    systemd.user.slices = mkOption {
      default = {};
      type = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig ] );
      description = "Definition of systemd per-user slice units.";
    };

    systemd.user.sockets = mkOption {
      default = {};
      type = with types; attrsOf (submodule [ { options = socketOptions; } unitConfig ] );
      description = "Definition of systemd per-user socket units.";
    };

    systemd.user.targets = mkOption {
      default = {};
      type = with types; attrsOf (submodule [ { options = targetOptions; } unitConfig] );
      description = "Definition of systemd per-user target units.";
    };

    systemd.user.timers = mkOption {
      default = {};
      type = with types; attrsOf (submodule [ { options = timerOptions; } unitConfig ] );
      description = "Definition of systemd per-user timer units.";
    };

    systemd.additionalUpstreamSystemUnits = mkOption {
      default = [ ];
      type = types.listOf types.str;
      example = [ "debug-shell.service" "systemd-quotacheck.service" ];
      description = ''
        Additional units shipped with systemd that shall be enabled.
      '';
    };

  };


  ###### implementation

  config = {

    warnings = concatLists (mapAttrsToList (name: service:
      optional (service.serviceConfig.Type or "" == "oneshot" && service.serviceConfig.Restart or "no" != "no")
        "Service ‘${name}.service’ with ‘Type=oneshot’ must have ‘Restart=no’") cfg.services);

    system.build.units = cfg.units;

    environment.systemPackages = [ systemd ];

    environment.etc = let
      # generate contents for /etc/systemd/system-generators from
      # systemd.generators and systemd.generator-packages
      generators = pkgs.runCommand "system-generators" {
          preferLocalBuild = true;
          packages = cfg.generator-packages;
        } ''
        mkdir -p $out
        for package in $packages
        do
          ln -s $package/lib/systemd/system-generators/* $out/
        done;
        ${concatStrings (mapAttrsToList (generator: target: "ln -s ${target} $out/${generator};\n") cfg.generators)}
      '';
    in ({
      "systemd/system".source = generateUnits "system" cfg.units upstreamSystemUnits upstreamSystemWants;

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

      "systemd/system.conf".text = ''
        [Manager]
        ${optionalString config.systemd.enableCgroupAccounting ''
          DefaultCPUAccounting=yes
          DefaultIOAccounting=yes
          DefaultBlockIOAccounting=yes
          DefaultMemoryAccounting=yes
          DefaultTasksAccounting=yes
        ''}
        ${config.systemd.extraConfig}
      '';

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

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

      "systemd/logind.conf".text = ''
        [Login]
        KillUserProcesses=${if config.services.logind.killUserProcesses then "yes" else "no"}
        HandleLidSwitch=${config.services.logind.lidSwitch}
        HandleLidSwitchDocked=${config.services.logind.lidSwitchDocked}
        HandleLidSwitchExternalPower=${config.services.logind.lidSwitchExternalPower}
        ${config.services.logind.extraConfig}
      '';

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

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

      "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}
      '';

      "systemd/system-generators" = { source = generators; };
    });

    services.dbus.enable = true;

    users.users.systemd-network.uid = config.ids.uids.systemd-network;
    users.groups.systemd-network.gid = config.ids.gids.systemd-network;
    users.users.systemd-resolve.uid = config.ids.uids.systemd-resolve;
    users.groups.systemd-resolve.gid = config.ids.gids.systemd-resolve;

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

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

    systemd.units =
         mapAttrs' (n: v: nameValuePair "${n}.path"    (pathToUnit    n v)) cfg.paths
      // mapAttrs' (n: v: nameValuePair "${n}.service" (serviceToUnit n v)) cfg.services
      // mapAttrs' (n: v: nameValuePair "${n}.slice"   (sliceToUnit   n v)) cfg.slices
      // mapAttrs' (n: v: nameValuePair "${n}.socket"  (socketToUnit  n v)) cfg.sockets
      // mapAttrs' (n: v: nameValuePair "${n}.target"  (targetToUnit  n v)) cfg.targets
      // mapAttrs' (n: v: nameValuePair "${n}.timer"   (timerToUnit   n v)) cfg.timers
      // 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 "${n}.path"    (pathToUnit    n v)) cfg.user.paths
      // mapAttrs' (n: v: nameValuePair "${n}.service" (serviceToUnit n v)) cfg.user.services
      // mapAttrs' (n: v: nameValuePair "${n}.slice"   (sliceToUnit   n v)) cfg.user.slices
      // mapAttrs' (n: v: nameValuePair "${n}.socket"  (socketToUnit  n v)) cfg.user.sockets
      // mapAttrs' (n: v: nameValuePair "${n}.target"  (targetToUnit  n v)) cfg.user.targets
      // mapAttrs' (n: v: nameValuePair "${n}.timer"   (timerToUnit   n v)) cfg.user.timers;

    system.requiredKernelConfig = map config.lib.kernelConfig.isEnabled
      [ "DEVTMPFS" "CGROUPS" "INOTIFY_USER" "SIGNALFD" "TIMERFD" "EPOLL" "NET"
        "SYSFS" "PROC_FS" "FHANDLE" "CRYPTO_USER_API_HASH" "CRYPTO_HMAC"
        "CRYPTO_SHA256" "DMIID" "AUTOFS4_FS" "TMPFS_POSIX_ACL"
        "TMPFS_XATTR" "SECCOMP"
      ];

    users.groups.systemd-journal.gid = config.ids.gids.systemd-journal;
    users.users.systemd-journal-gateway.uid = config.ids.uids.systemd-journal-gateway;
    users.groups.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.enable && service.startAt != []) cfg.services);

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

    # Some overrides to upstream units.
    systemd.services."systemd-backlight@".restartIfChanged = false;
    systemd.services."systemd-fsck@".restartIfChanged = false;
    systemd.services."systemd-fsck@".path = [ config.system.path ];
    systemd.services."user@".restartIfChanged = false;
    systemd.services.systemd-journal-flush.restartIfChanged = false;
    systemd.services.systemd-random-seed.restartIfChanged = false;
    systemd.services.systemd-remount-fs.restartIfChanged = false;
    systemd.services.systemd-update-utmp.restartIfChanged = false;
    systemd.services.systemd-user-sessions.restartIfChanged = false; # Restart kills all active sessions.
    systemd.services.systemd-udev-settle.restartIfChanged = false; # Causes long delays in nixos-rebuild
    # Restarting systemd-logind breaks X11
    # - upstream commit: https://cgit.freedesktop.org/xorg/xserver/commit/?id=dc48bd653c7e101
    # - systemd announcement: https://github.com/systemd/systemd/blob/22043e4317ecd2bc7834b48a6d364de76bb26d91/NEWS#L103-L112
    # - this might be addressed in the future by xorg
    #systemd.services.systemd-logind.restartTriggers = [ config.environment.etc."systemd/logind.conf".source ];
    systemd.services.systemd-logind.restartIfChanged = false;
    systemd.services.systemd-logind.stopIfChanged = false;
    # The user-runtime-dir@ service is managed by systemd-logind we should not touch it or else we break the users' sessions.
    systemd.services."user-runtime-dir@".stopIfChanged = false;
    systemd.services."user-runtime-dir@".restartIfChanged = false;
    systemd.services.systemd-journald.restartTriggers = [ config.environment.etc."systemd/journald.conf".source ];
    systemd.services.systemd-journald.stopIfChanged = false;
    systemd.targets.local-fs.unitConfig.X-StopOnReconfiguration = true;
    systemd.targets.remote-fs.unitConfig.X-StopOnReconfiguration = true;
    systemd.targets.network-online.wantedBy = [ "multi-user.target" ];
    systemd.services.systemd-binfmt.wants = [ "proc-sys-fs-binfmt_misc.mount" ];

    # Don't bother with certain units in containers.
    systemd.services.systemd-remount-fs.unitConfig.ConditionVirtualization = "!container";
    systemd.services.systemd-random-seed.unitConfig.ConditionVirtualization = "!container";

  };

  # FIXME: Remove these eventually.
  imports =
    [ (mkRenamedOptionModule [ "boot" "systemd" "sockets" ] [ "systemd" "sockets" ])
      (mkRenamedOptionModule [ "boot" "systemd" "targets" ] [ "systemd" "targets" ])
      (mkRenamedOptionModule [ "boot" "systemd" "services" ] [ "systemd" "services" ])
    ];

}