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

with utils.systemdUtils.unitOptions;
with utils.systemdUtils.lib;
with lib;

let
  cfg = config.systemd.nspawn;

  checkExec = checkUnitConfig "Exec" [
    (assertOnlyFields [
      "Boot" "ProcessTwo" "Parameters" "Environment" "User" "WorkingDirectory"
      "PivotRoot" "Capability" "DropCapability" "NoNewPrivileges" "KillSignal"
      "Personality" "MachineId" "PrivateUsers" "NotifyReady" "SystemCallFilter"
      "LimitCPU" "LimitFSIZE" "LimitDATA" "LimitSTACK" "LimitCORE" "LimitRSS"
      "LimitNOFILE" "LimitAS" "LimitNPROC" "LimitMEMLOCK" "LimitLOCKS"
      "LimitSIGPENDING" "LimitMSGQUEUE" "LimitNICE" "LimitRTPRIO" "LimitRTTIME"
      "OOMScoreAdjust" "CPUAffinity" "Hostname" "ResolvConf" "Timezone"
      "LinkJournal"
    ])
    (assertValueOneOf "Boot" boolValues)
    (assertValueOneOf "ProcessTwo" boolValues)
    (assertValueOneOf "NotifyReady" boolValues)
  ];

  checkFiles = checkUnitConfig "Files" [
    (assertOnlyFields [
      "ReadOnly" "Volatile" "Bind" "BindReadOnly" "TemporaryFileSystem"
      "Overlay" "OverlayReadOnly" "PrivateUsersChown"
    ])
    (assertValueOneOf "ReadOnly" boolValues)
    (assertValueOneOf "Volatile" (boolValues ++ [ "state" ]))
    (assertValueOneOf "PrivateUsersChown" boolValues)
  ];

  checkNetwork = checkUnitConfig "Network" [
    (assertOnlyFields [
      "Private" "VirtualEthernet" "VirtualEthernetExtra" "Interface" "MACVLAN"
      "IPVLAN" "Bridge" "Zone" "Port"
    ])
    (assertValueOneOf "Private" boolValues)
    (assertValueOneOf "VirtualEthernet" boolValues)
  ];

  instanceOptions = {
    options = sharedOptions // {
      execConfig = mkOption {
        default = {};
        example = { Parameters = "/bin/sh"; };
        type = types.addCheck (types.attrsOf unitOption) checkExec;
        description = ''
          Each attribute in this set specifies an option in the
          <literal>[Exec]</literal> section of this unit. See
          <citerefentry><refentrytitle>systemd.nspawn</refentrytitle>
          <manvolnum>5</manvolnum></citerefentry> for details.
        '';
      };

      filesConfig = mkOption {
        default = {};
        example = { Bind = [ "/home/alice" ]; };
        type = types.addCheck (types.attrsOf unitOption) checkFiles;
        description = ''
          Each attribute in this set specifies an option in the
          <literal>[Files]</literal> section of this unit. See
          <citerefentry><refentrytitle>systemd.nspawn</refentrytitle>
          <manvolnum>5</manvolnum></citerefentry> for details.
        '';
      };

      networkConfig = mkOption {
        default = {};
        example = { Private = false; };
        type = types.addCheck (types.attrsOf unitOption) checkNetwork;
        description = ''
          Each attribute in this set specifies an option in the
          <literal>[Network]</literal> section of this unit. See
          <citerefentry><refentrytitle>systemd.nspawn</refentrytitle>
          <manvolnum>5</manvolnum></citerefentry> for details.
        '';
      };
    };

  };

  instanceToUnit = name: def:
    let base = {
      text = ''
        [Exec]
        ${attrsToSection def.execConfig}

        [Files]
        ${attrsToSection def.filesConfig}

        [Network]
        ${attrsToSection def.networkConfig}
      '';
    } // def;
    in base // { unit = makeUnit name base; };

in {

  options = {

    systemd.nspawn = mkOption {
      default = {};
      type = with types; attrsOf (submodule instanceOptions);
      description = "Definition of systemd-nspawn configurations.";
    };

  };

  config =
    let
      units = mapAttrs' (n: v: let nspawnFile = "${n}.nspawn"; in nameValuePair nspawnFile (instanceToUnit nspawnFile v)) cfg;
    in
      mkMerge [
        (mkIf (cfg != {}) {
          environment.etc."systemd/nspawn".source = mkIf (cfg != {}) (generateUnits' false "nspawn" units [] []);
        })
        {
          systemd.targets.multi-user.wants = [ "machines.target" ];

          # Workaround for https://github.com/NixOS/nixpkgs/pull/67232#issuecomment-531315437 and https://github.com/systemd/systemd/issues/13622
          # Once systemd fixes this upstream, we can re-enable -U
          systemd.services."systemd-nspawn@".serviceConfig.ExecStart = [
            ""  # deliberately empty. signals systemd to override the ExecStart
            # Only difference between upstream is that we do not pass the -U flag
            "${config.systemd.package}/bin/systemd-nspawn --quiet --keep-unit --boot --link-journal=try-guest --network-veth --settings=override --machine=%i"
          ];
        }
      ];
}