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

with utils;
with systemdUtils.unitOptions;
with lib;

let

  cfg = config.systemd;

  inherit (systemdUtils.lib)
    generateUnits
    targetToUnit
    serviceToUnit
    socketToUnit
    timerToUnit
    pathToUnit
    mountToUnit
    automountToUnit
    sliceToUnit;

  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"
    ] ++ optionals cfg.package.withCryptsetup [
      "cryptsetup.target"
      "cryptsetup-pre.target"
      "remote-cryptsetup.target"
    ] ++ [
      "sigpwr.target"
      "timers.target"
      "paths.target"
      "rpcbind.target"

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

      # Udev.
      "systemd-tmpfiles-setup-dev-early.service"
      "systemd-udevd-control.socket"
      "systemd-udevd-kernel.socket"
      "systemd-udevd.service"
      "systemd-udev-settle.service"
      ] ++ (optional (!config.boot.isContainer) "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"

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

      # Filesystems.
      "systemd-fsck@.service"
      "systemd-fsck-root.service"
      "systemd-growfs@.service"
      "systemd-growfs-root.service"
      "systemd-remount-fs.service"
      "systemd-pstore.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"
      ] ++ (optional (!config.boot.isContainer) "sys-kernel-config.mount") ++ [
      "sys-kernel-debug.mount"

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

      # Hibernate / suspend.
      "hibernate.target"
      "suspend.target"
      "suspend-then-hibernate.target"
      "sleep.target"
      "hybrid-sleep.target"
      "systemd-hibernate.service"
      "systemd-hybrid-sleep.service"
      "systemd-suspend.service"
      "systemd-suspend-then-hibernate.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"
    ] ++ lib.optional cfg.package.withUtmp "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"
    ] ++ optionals cfg.package.withImportd [
      "systemd-importd.service"
    ] ++ optionals cfg.package.withMachined [
      "machine.slice"
      "machines.target"
      "systemd-machined.service"
    ] ++ [
      "systemd-nspawn@.service"

      # Misc.
      "systemd-sysctl.service"
    ] ++ optionals cfg.package.withTimedated [
      "dbus-org.freedesktop.timedate1.service"
      "systemd-timedated.service"
    ] ++ optionals cfg.package.withLocaled [
      "dbus-org.freedesktop.locale1.service"
      "systemd-localed.service"
    ] ++ optionals cfg.package.withHostnamed [
      "dbus-org.freedesktop.hostname1.service"
      "systemd-hostnamed.service"
    ] ++ optionals cfg.package.withPortabled [
      "dbus-org.freedesktop.portable1.service"
      "systemd-portabled.service"
    ] ++ [
      "systemd-exit.service"
      "systemd-update-done.service"
    ] ++ cfg.additionalUpstreamSystemUnits;

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

  proxy_env = config.networking.proxy.envVars;

in

{
  ###### interface

  options.systemd = {

    package = mkPackageOption pkgs "systemd" {};

    units = mkOption {
      description = "Definition of systemd units; see {manpage}`systemd.unit(5)`.";
      default = {};
      type = systemdUtils.types.units;
    };

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

    targets = mkOption {
      default = {};
      type = systemdUtils.types.targets;
      description = "Definition of systemd target units; see {manpage}`systemd.target(5)`";
    };

    services = mkOption {
      default = {};
      type = systemdUtils.types.services;
      description = "Definition of systemd service units; see {manpage}`systemd.service(5)`.";
    };

    sockets = mkOption {
      default = {};
      type = systemdUtils.types.sockets;
      description = "Definition of systemd socket units; see {manpage}`systemd.socket(5)`.";
    };

    timers = mkOption {
      default = {};
      type = systemdUtils.types.timers;
      description = "Definition of systemd timer units; see {manpage}`systemd.timer(5)`.";
    };

    paths = mkOption {
      default = {};
      type = systemdUtils.types.paths;
      description = "Definition of systemd path units; see {manpage}`systemd.path(5)`.";
    };

    mounts = mkOption {
      default = [];
      type = systemdUtils.types.mounts;
      description = ''
        Definition of systemd mount units; see {manpage}`systemd.mount(5)`.

        This is a list instead of an attrSet, because systemd mandates
        the names to be derived from the `where` attribute.
      '';
    };

    automounts = mkOption {
      default = [];
      type = systemdUtils.types.automounts;
      description = ''
        Definition of systemd automount units; see {manpage}`systemd.automount(5)`.

        This is a list instead of an attrSet, because systemd mandates
        the names to be derived from the `where` attribute.
      '';
    };

    slices = mkOption {
      default = {};
      type = systemdUtils.types.slices;
      description = "Definition of slice configurations; see {manpage}`systemd.slice(5)`.";
    };

    generators = mkOption {
      type = types.attrsOf types.path;
      default = {};
      example = { systemd-gpt-auto-generator = "/dev/null"; };
      description = ''
        Definition of systemd generators; see {manpage}`systemd.generator(5)`.

        For each `NAME = VALUE` pair of the attrSet, a link is generated from
        `/etc/systemd/system-generators/NAME` to `VALUE`.
      '';
    };

    shutdown = mkOption {
      type = types.attrsOf types.path;
      default = {};
      description = ''
        Definition of systemd shutdown executables.
        For each `NAME = VALUE` pair of the attrSet, a link is generated from
        `/etc/systemd/system-shutdown/NAME` to `VALUE`.
      '';
    };

    defaultUnit = mkOption {
      default = "multi-user.target";
      type = types.str;
      description = ''
        Default unit started when the system boots; see {manpage}`systemd.special(7)`.
      '';
    };

    ctrlAltDelUnit = mkOption {
      default = "reboot.target";
      type = types.str;
      example = "poweroff.target";
      description = ''
        Target that should be started when Ctrl-Alt-Delete is pressed;
        see {manpage}`systemd.special(7)`.
      '';
    };

    globalEnvironment = mkOption {
      type = with types; attrsOf (nullOr (oneOf [ str path package ]));
      default = {};
      example = { TZ = "CET"; };
      description = ''
        Environment variables passed to *all* systemd units.
      '';
    };

    managerEnvironment = mkOption {
      type = with types; attrsOf (nullOr (oneOf [ str path package ]));
      default = {};
      example = { SYSTEMD_LOG_LEVEL = "debug"; };
      description = ''
        Environment variables of PID 1. These variables are
        *not* passed to started units.
      '';
    };

    enableCgroupAccounting = mkOption {
      default = true;
      type = types.bool;
      description = ''
        Whether to enable cgroup accounting; see {manpage}`cgroups(7)`.
      '';
    };

    enableUnifiedCgroupHierarchy = mkOption {
      default = true;
      type = types.bool;
      description = ''
        Whether to enable the unified cgroup hierarchy (cgroupsv2); see {manpage}`cgroups(7)`.
      '';
    };

    extraConfig = mkOption {
      default = "";
      type = types.lines;
      example = "DefaultLimitCORE=infinity";
      description = ''
        Extra config options for systemd. See {manpage}`systemd-system.conf(5)` man page
        for available options.
      '';
    };

    sleep.extraConfig = mkOption {
      default = "";
      type = types.lines;
      example = "HibernateDelaySec=1h";
      description = ''
        Extra config options for systemd sleep state logic.
        See {manpage}`sleep.conf.d(5)` man page for available options.
      '';
    };

    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.
      '';
    };

    suppressedSystemUnits = mkOption {
      default = [ ];
      type = types.listOf types.str;
      example = [ "systemd-backlight@.service" ];
      description = ''
        A list of units to skip when generating system systemd configuration directory. This has
        priority over upstream units, {option}`systemd.units`, and
        {option}`systemd.additionalUpstreamSystemUnits`. The main purpose of this is to
        prevent a upstream systemd unit from being added to the initrd with any modifications made to it
        by other NixOS modules.
      '';
    };

    watchdog.device = mkOption {
      type = types.nullOr types.path;
      default = null;
      example = "/dev/watchdog";
      description = ''
        The path to a hardware watchdog device which will be managed by systemd.
        If not specified, systemd will default to `/dev/watchdog`.
      '';
    };

    watchdog.runtimeTime = mkOption {
      type = types.nullOr types.str;
      default = null;
      example = "30s";
      description = ''
        The amount of time which can elapse before a watchdog hardware device
        will automatically reboot the system.

        Valid time units include "ms", "s", "min", "h", "d", and "w";
        see {manpage}`systemd.time(7)`.
      '';
    };

    watchdog.rebootTime = mkOption {
      type = types.nullOr types.str;
      default = null;
      example = "10m";
      description = ''
        The amount of time which can elapse after a reboot has been triggered
        before a watchdog hardware device will automatically reboot the system.
        If left `null`, systemd will use its default of 10 minutes;
        see {manpage}`systemd-system.conf(5)`.

        Valid time units include "ms", "s", "min", "h", "d", and "w";
        see also {manpage}`systemd.time(7)`.
      '';
    };

    watchdog.kexecTime = mkOption {
      type = types.nullOr types.str;
      default = null;
      example = "10m";
      description = ''
        The amount of time which can elapse when `kexec` is being executed before
        a watchdog hardware device will automatically reboot the system. This
        option should only be enabled if `reloadTime` is also enabled;
        see {manpage}`kexec(8)`.

        Valid time units include "ms", "s", "min", "h", "d", and "w";
        see also {manpage}`systemd.time(7)`.
      '';
    };
  };


  ###### implementation

  config = {

    warnings = let
      mkOneNetOnlineWarn = typeStr: name: def: lib.optional
        (lib.elem "network-online.target" def.after && !(lib.elem "network-online.target" (def.wants ++ def.requires ++ def.bindsTo)))
        "${name}.${typeStr} is ordered after 'network-online.target' but doesn't depend on it";
      mkNetOnlineWarns = typeStr: defs: lib.concatLists (lib.mapAttrsToList (mkOneNetOnlineWarn typeStr) defs);
      mkMountNetOnlineWarns = typeStr: defs: lib.concatLists (map (m: mkOneNetOnlineWarn typeStr m.what m) defs);
    in concatLists (
      mapAttrsToList
        (name: service:
          let
            type = service.serviceConfig.Type or "";
            restart = service.serviceConfig.Restart or "no";
            hasDeprecated = builtins.hasAttr "StartLimitInterval" service.serviceConfig;
          in
            concatLists [
              (optional (type == "oneshot" && (restart == "always" || restart == "on-success"))
                "Service '${name}.service' with 'Type=oneshot' cannot have 'Restart=always' or 'Restart=on-success'"
              )
              (optional hasDeprecated
                "Service '${name}.service' uses the attribute 'StartLimitInterval' in the Service section, which is deprecated. See https://github.com/NixOS/nixpkgs/issues/45786."
              )
              (optional (service.reloadIfChanged && service.reloadTriggers != [])
                "Service '${name}.service' has both 'reloadIfChanged' and 'reloadTriggers' set. This is probably not what you want, because 'reloadTriggers' behave the same whay as 'restartTriggers' if 'reloadIfChanged' is set."
              )
            ]
        )
        cfg.services
    )
    ++ (mkNetOnlineWarns "target" cfg.targets)
    ++ (mkNetOnlineWarns "service" cfg.services)
    ++ (mkNetOnlineWarns "socket" cfg.sockets)
    ++ (mkNetOnlineWarns "timer" cfg.timers)
    ++ (mkNetOnlineWarns "path" cfg.paths)
    ++ (mkMountNetOnlineWarns "mount" cfg.mounts)
    ++ (mkMountNetOnlineWarns "automount" cfg.automounts)
    ++ (mkNetOnlineWarns "slice" cfg.slices);

    assertions = concatLists (
      mapAttrsToList
        (name: service:
          map (message: {
            assertion = false;
            inherit message;
          }) (concatLists [
            (optional ((builtins.elem "network-interfaces.target" service.after) || (builtins.elem "network-interfaces.target" service.wants))
              "Service '${name}.service' is using the deprecated target network-interfaces.target, which no longer exists. Using network.target is recommended instead."
            )
          ])
        )
        cfg.services
    );

    system.build.units = cfg.units;

    system.nssModules = [ cfg.package.out ];
    system.nssDatabases = {
      hosts = (mkMerge [
        (mkOrder 400 ["mymachines"]) # 400 to ensure it comes before resolve (which is mkBefore'd)
        (mkOrder 999 ["myhostname"]) # after files (which is 998), but before regular nss modules
      ]);
      passwd = (mkMerge [
        (mkAfter [ "systemd" ])
      ]);
      group = (mkMerge [
        (mkAfter [ "[success=merge] systemd" ]) # need merge so that NSS won't stop at file-based groups
      ]);
    };

    environment.systemPackages = [ cfg.package ];

    environment.etc = let
      # generate contents for /etc/systemd/system-${type} from attrset of links and packages
      hooks = type: links: pkgs.runCommand "system-${type}" {
          preferLocalBuild = true;
          packages = cfg.packages;
      } ''
        set -e
        mkdir -p $out
        for package in $packages
        do
          for hook in $package/lib/systemd/system-${type}/*
          do
            ln -s $hook $out/
          done
        done
        ${concatStrings (mapAttrsToList (exec: target: "ln -s ${target} $out/${exec};\n") links)}
      '';

      enabledUpstreamSystemUnits = filter (n: ! elem n cfg.suppressedSystemUnits) upstreamSystemUnits;
      enabledUnits = filterAttrs (n: v: ! elem n cfg.suppressedSystemUnits) cfg.units;

    in ({
      "systemd/system".source = generateUnits {
        type = "system";
        units = enabledUnits;
        upstreamUnits = enabledUpstreamSystemUnits;
        upstreamWants = upstreamSystemWants;
      };

      "systemd/system.conf".text = ''
        [Manager]
        ManagerEnvironment=${lib.concatStringsSep " " (lib.mapAttrsToList (n: v: "${n}=${lib.escapeShellArg v}") cfg.managerEnvironment)}
        ${optionalString cfg.enableCgroupAccounting ''
          DefaultCPUAccounting=yes
          DefaultIOAccounting=yes
          DefaultBlockIOAccounting=yes
          DefaultIPAccounting=yes
        ''}
        DefaultLimitCORE=infinity
        ${optionalString (cfg.watchdog.device != null) ''
          WatchdogDevice=${cfg.watchdog.device}
        ''}
        ${optionalString (cfg.watchdog.runtimeTime != null) ''
          RuntimeWatchdogSec=${cfg.watchdog.runtimeTime}
        ''}
        ${optionalString (cfg.watchdog.rebootTime != null) ''
          RebootWatchdogSec=${cfg.watchdog.rebootTime}
        ''}
        ${optionalString (cfg.watchdog.kexecTime != null) ''
          KExecWatchdogSec=${cfg.watchdog.kexecTime}
        ''}

        ${cfg.extraConfig}
      '';

      "systemd/sleep.conf".text = ''
        [Sleep]
        ${cfg.sleep.extraConfig}
      '';

      "systemd/system-generators" = { source = hooks "generators" cfg.generators; };
      "systemd/system-shutdown" = { source = hooks "shutdown" cfg.shutdown; };
    });

    services.dbus.enable = true;

    users.users.systemd-network = {
      uid = config.ids.uids.systemd-network;
      group = "systemd-network";
    };
    users.groups.systemd-network.gid = config.ids.gids.systemd-network;
    users.users.systemd-resolve = {
      uid = config.ids.uids.systemd-resolve;
      group = "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;
      };

    # This target only exists so that services ordered before sysinit.target
    # are restarted in the correct order, notably BEFORE the other services,
    # when switching configurations.
    systemd.targets.sysinit-reactivation = {
      description = "Reactivate sysinit units";
    };

    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);

      # Environment of PID 1
      systemd.managerEnvironment = {
        # Doesn't contain systemd itself - everything works so it seems to use the compiled-in value for its tools
        # util-linux is needed for the main fsck utility wrapping the fs-specific ones
        PATH = lib.makeBinPath (config.system.fsPackages ++ [cfg.package.util-linux]);
        LOCALE_ARCHIVE = "/run/current-system/sw/lib/locale/locale-archive";
        TZDIR = "/etc/zoneinfo";
        # If SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable
        SYSTEMD_UNIT_PATH = lib.mkIf (config.boot.extraSystemdUnitPaths != []) "${builtins.concatStringsSep ":" config.boot.extraSystemdUnitPaths}:";
      };


    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" "AUTOFS_FS" "TMPFS_POSIX_ACL"
        "TMPFS_XATTR" "SECCOMP"
      ];

    # 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);

    # Some overrides to upstream units.
    systemd.services."systemd-backlight@".restartIfChanged = false;
    systemd.services."systemd-fsck@".restartIfChanged = false;
    systemd.services."systemd-fsck@".path = [ pkgs.util-linux ] ++ config.system.fsPackages;
    systemd.services."systemd-makefs@" = {
      restartIfChanged = false;
      path = [ pkgs.util-linux ] ++ config.system.fsPackages;
      # Since there is no /etc/systemd/system/systemd-makefs@.service
      # file, the units generated in /run/systemd/generator would
      # override anything we put here. But by forcing the use of a
      # drop-in in /etc, it does apply.
      overrideStrategy = "asDropin";
    };
    systemd.services."systemd-mkswap@" = {
      restartIfChanged = false;
      path = [ pkgs.util-linux ];
      overrideStrategy = "asDropin";
    };
    systemd.services.systemd-random-seed.restartIfChanged = false;
    systemd.services.systemd-remount-fs.restartIfChanged = false;
    systemd.services.systemd-update-utmp.restartIfChanged = false;
    systemd.services.systemd-udev-settle.restartIfChanged = false; # Causes long delays in nixos-rebuild
    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-importd.environment = proxy_env;
    systemd.services.systemd-pstore.wantedBy = [ "sysinit.target" ]; # see #81138

    # NixOS has kernel modules in a different location, so override that here.
    systemd.services.kmod-static-nodes.unitConfig.ConditionFileNotEmpty = [
      ""  # required to unset the previous value!
      "/run/booted-system/kernel-modules/lib/modules/%v/modules.devname"
    ];

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

    # Increase numeric PID range (set directly instead of copying a one-line file from systemd)
    # https://github.com/systemd/systemd/pull/12226
    boot.kernel.sysctl."kernel.pid_max" = mkIf pkgs.stdenv.is64bit (lib.mkDefault 4194304);

    boot.kernelParams = optional (!cfg.enableUnifiedCgroupHierarchy) "systemd.unified_cgroup_hierarchy=0";

    # Avoid potentially degraded system state due to
    # "Userspace Out-Of-Memory (OOM) Killer was skipped because of a failed condition check (ConditionControlGroupController=v2)."
    systemd.oomd.enable = mkIf (!cfg.enableUnifiedCgroupHierarchy) false;

    services.logrotate.settings = {
      "/var/log/btmp" = mapAttrs (_: mkDefault) {
        frequency = "monthly";
        rotate = 1;
        create = "0660 root ${config.users.groups.utmp.name}";
        minsize = "1M";
      };
      "/var/log/wtmp" = mapAttrs (_: mkDefault) {
        frequency = "monthly";
        rotate = 1;
        create = "0664 root ${config.users.groups.utmp.name}";
        minsize = "1M";
      };
    };
  };

  # FIXME: Remove these eventually.
  imports =
    [ (mkRenamedOptionModule [ "boot" "systemd" "sockets" ] [ "systemd" "sockets" ])
      (mkRenamedOptionModule [ "boot" "systemd" "targets" ] [ "systemd" "targets" ])
      (mkRenamedOptionModule [ "boot" "systemd" "services" ] [ "systemd" "services" ])
      (mkRenamedOptionModule [ "jobs" ] [ "systemd" "services" ])
      (mkRemovedOptionModule [ "systemd" "generator-packages" ] "Use systemd.packages instead.")
    ];
}