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

let
  inherit (lib)
    any
    attrNames
    attrValues
    concatMap
    concatStrings
    elem
    filter
    filterAttrs
    flatten
    flip
    foldr
    getAttr
    hasAttr
    id
    length
    listToAttrs
    literalExpression
    mapAttrs'
    mapAttrsToList
    match
    mkAliasOptionModuleMD
    mkDefault
    mkIf
    mkMerge
    mkOption
    mkRenamedOptionModule
    optional
    optionals
    sort
    stringAfter
    stringLength
    trace
    types
    xor
    ;

  ids = config.ids;
  cfg = config.users;

  # Check whether a password hash will allow login.
  allowsLogin = hash:
    hash == "" # login without password
    || !(lib.elem hash
      [ null   # password login disabled
        "!"    # password login disabled
        "!!"   # a variant of "!"
        "*"    # password unset
      ]);

  passwordDescription = ''
    The options {option}`hashedPassword`,
    {option}`password` and {option}`hashedPasswordFile`
    controls what password is set for the user.
    {option}`hashedPassword` overrides both
    {option}`password` and {option}`hashedPasswordFile`.
    {option}`password` overrides {option}`hashedPasswordFile`.
    If none of these three options are set, no password is assigned to
    the user, and the user will not be able to do password logins.
    If the option {option}`users.mutableUsers` is true, the
    password defined in one of the three options will only be set when
    the user is created for the first time. After that, you are free to
    change the password with the ordinary user management commands. If
    {option}`users.mutableUsers` is false, you cannot change
    user passwords, they will always be set according to the password
    options.
  '';

  hashedPasswordDescription = ''
    To generate a hashed password run `mkpasswd`.

    If set to an empty string (`""`), this user will
    be able to log in without being asked for a password (but not via remote
    services such as SSH, or indirectly via {command}`su` or
    {command}`sudo`). This should only be used for e.g. bootable
    live systems. Note: this is different from setting an empty password,
    which can be achieved using {option}`users.users.<name?>.password`.

    If set to `null` (default) this user will not
    be able to log in using a password (i.e. via {command}`login`
    command).
  '';

  userOpts = { name, config, ... }: {

    options = {

      name = mkOption {
        type = types.passwdEntry types.str;
        apply = x: assert (stringLength x < 32 || abort "Username '${x}' is longer than 31 characters which is not allowed!"); x;
        description = ''
          The name of the user account. If undefined, the name of the
          attribute set will be used.
        '';
      };

      description = mkOption {
        type = types.passwdEntry types.str;
        default = "";
        example = "Alice Q. User";
        description = ''
          A short description of the user account, typically the
          user's full name.  This is actually the “GECOS” or “comment”
          field in {file}`/etc/passwd`.
        '';
      };

      uid = mkOption {
        type = with types; nullOr int;
        default = null;
        description = ''
          The account UID. If the UID is null, a free UID is picked on
          activation.
        '';
      };

      isSystemUser = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Indicates if the user is a system user or not. This option
          only has an effect if {option}`uid` is
          {option}`null`, in which case it determines whether
          the user's UID is allocated in the range for system users
          (below 1000) or in the range for normal users (starting at
          1000).
          Exactly one of `isNormalUser` and
          `isSystemUser` must be true.
        '';
      };

      isNormalUser = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Indicates whether this is an account for a “real” user.
          This automatically sets {option}`group` to `users`,
          {option}`createHome` to `true`,
          {option}`home` to {file}`/home/«username»`,
          {option}`useDefaultShell` to `true`,
          and {option}`isSystemUser` to `false`.
          Exactly one of `isNormalUser` and `isSystemUser` must be true.
        '';
      };

      group = mkOption {
        type = types.str;
        apply = x: assert (stringLength x < 32 || abort "Group name '${x}' is longer than 31 characters which is not allowed!"); x;
        default = "";
        description = "The user's primary group.";
      };

      extraGroups = mkOption {
        type = types.listOf types.str;
        default = [];
        description = "The user's auxiliary groups.";
      };

      home = mkOption {
        type = types.passwdEntry types.path;
        default = "/var/empty";
        description = "The user's home directory.";
      };

      homeMode = mkOption {
        type = types.strMatching "[0-7]{1,5}";
        default = "700";
        description = "The user's home directory mode in numeric format. See chmod(1). The mode is only applied if {option}`users.users.<name>.createHome` is true.";
      };

      cryptHomeLuks = mkOption {
        type = with types; nullOr str;
        default = null;
        description = ''
          Path to encrypted luks device that contains
          the user's home directory.
        '';
      };

      pamMount = mkOption {
        type = with types; attrsOf str;
        default = {};
        description = ''
          Attributes for user's entry in
          {file}`pam_mount.conf.xml`.
          Useful attributes might include `path`,
          `options`, `fstype`, and `server`.
          See <https://pam-mount.sourceforge.net/pam_mount.conf.5.html>
          for more information.
        '';
      };

      shell = mkOption {
        type = types.nullOr (types.either types.shellPackage (types.passwdEntry types.path));
        default = pkgs.shadow;
        defaultText = literalExpression "pkgs.shadow";
        example = literalExpression "pkgs.bashInteractive";
        description = ''
          The path to the user's shell. Can use shell derivations,
          like `pkgs.bashInteractive`. Don’t
          forget to enable your shell in
          `programs` if necessary,
          like `programs.zsh.enable = true;`.
        '';
      };

      ignoreShellProgramCheck = mkOption {
        type = types.bool;
        default = false;
        description = ''
          By default, nixos will check that programs.SHELL.enable is set to
          true if the user has a custom shell specified. If that behavior isn't
          required and there are custom overrides in place to make sure that the
          shell is functional, set this to true.
        '';
      };

      subUidRanges = mkOption {
        type = with types; listOf (submodule subordinateUidRange);
        default = [];
        example = [
          { startUid = 1000; count = 1; }
          { startUid = 100001; count = 65534; }
        ];
        description = ''
          Subordinate user ids that user is allowed to use.
          They are set into {file}`/etc/subuid` and are used
          by `newuidmap` for user namespaces.
        '';
      };

      subGidRanges = mkOption {
        type = with types; listOf (submodule subordinateGidRange);
        default = [];
        example = [
          { startGid = 100; count = 1; }
          { startGid = 1001; count = 999; }
        ];
        description = ''
          Subordinate group ids that user is allowed to use.
          They are set into {file}`/etc/subgid` and are used
          by `newgidmap` for user namespaces.
        '';
      };

      autoSubUidGidRange = mkOption {
        type = types.bool;
        default = false;
        example = true;
        description = ''
          Automatically allocate subordinate user and group ids for this user.
          Allocated range is currently always of size 65536.
        '';
      };

      createHome = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether to create the home directory and ensure ownership as well as
          permissions to match the user.
        '';
      };

      useDefaultShell = mkOption {
        type = types.bool;
        default = false;
        description = ''
          If true, the user's shell will be set to
          {option}`users.defaultUserShell`.
        '';
      };

      hashedPassword = mkOption {
        type = with types; nullOr (passwdEntry str);
        default = null;
        description = ''
          Specifies the hashed password for the user.
          ${passwordDescription}
          ${hashedPasswordDescription}
        '';
      };

      password = mkOption {
        type = with types; nullOr str;
        default = null;
        description = ''
          Specifies the (clear text) password for the user.
          Warning: do not set confidential information here
          because it is world-readable in the Nix store. This option
          should only be used for public accounts.
          ${passwordDescription}
        '';
      };

      hashedPasswordFile = mkOption {
        type = with types; nullOr str;
        default = cfg.users.${name}.passwordFile;
        defaultText = literalExpression "null";
        description = ''
          The full path to a file that contains the hash of the user's
          password. The password file is read on each system activation. The
          file should contain exactly one line, which should be the password in
          an encrypted form that is suitable for the `chpasswd -e` command.
          ${passwordDescription}
        '';
      };

      passwordFile = mkOption {
        type = with types; nullOr str;
        default = null;
        visible = false;
        description = "Deprecated alias of hashedPasswordFile";
      };

      initialHashedPassword = mkOption {
        type = with types; nullOr (passwdEntry str);
        default = null;
        description = ''
          Specifies the initial hashed password for the user, i.e. the
          hashed password assigned if the user does not already
          exist. If {option}`users.mutableUsers` is true, the
          password can be changed subsequently using the
          {command}`passwd` command. Otherwise, it's
          equivalent to setting the {option}`hashedPassword` option.

          Note that the {option}`hashedPassword` option will override
          this option if both are set.

          ${hashedPasswordDescription}
        '';
      };

      initialPassword = mkOption {
        type = with types; nullOr str;
        default = null;
        description = ''
          Specifies the initial password for the user, i.e. the
          password assigned if the user does not already exist. If
          {option}`users.mutableUsers` is true, the password
          can be changed subsequently using the
          {command}`passwd` command. Otherwise, it's
          equivalent to setting the {option}`password`
          option. The same caveat applies: the password specified here
          is world-readable in the Nix store, so it should only be
          used for guest accounts or passwords that will be changed
          promptly.

          Note that the {option}`password` option will override this
          option if both are set.
        '';
      };

      packages = mkOption {
        type = types.listOf types.package;
        default = [];
        example = literalExpression "[ pkgs.firefox pkgs.thunderbird ]";
        description = ''
          The set of packages that should be made available to the user.
          This is in contrast to {option}`environment.systemPackages`,
          which adds packages to all users.
        '';
      };

      expires = mkOption {
        type = types.nullOr (types.strMatching "[[:digit:]]{4}-[[:digit:]]{2}-[[:digit:]]{2}");
        default = null;
        description = ''
          Set the date on which the user's account will no longer be
          accessible. The date is expressed in the format YYYY-MM-DD, or null
          to disable the expiry.
          A user whose account is locked must contact the system
          administrator before being able to use the system again.
        '';
      };

      linger = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether to enable lingering for this user. If true, systemd user
          units will start at boot, rather than starting at login and stopping
          at logout. This is the declarative equivalent of running
          `loginctl enable-linger` for this user.

          If false, user units will not be started until the user logs in, and
          may be stopped on logout depending on the settings in `logind.conf`.
        '';
      };
    };

    config = mkMerge
      [ { name = mkDefault name;
          shell = mkIf config.useDefaultShell (mkDefault cfg.defaultUserShell);
        }
        (mkIf config.isNormalUser {
          group = mkDefault "users";
          createHome = mkDefault true;
          home = mkDefault "/home/${config.name}";
          homeMode = mkDefault "700";
          useDefaultShell = mkDefault true;
          isSystemUser = mkDefault false;
        })
        # If !mutableUsers, setting ‘initialPassword’ is equivalent to
        # setting ‘password’ (and similarly for hashed passwords).
        (mkIf (!cfg.mutableUsers && config.initialPassword != null) {
          password = mkDefault config.initialPassword;
        })
        (mkIf (!cfg.mutableUsers && config.initialHashedPassword != null) {
          hashedPassword = mkDefault config.initialHashedPassword;
        })
        (mkIf (config.isNormalUser && config.subUidRanges == [] && config.subGidRanges == []) {
          autoSubUidGidRange = mkDefault true;
        })
      ];

  };

  groupOpts = { name, config, ... }: {

    options = {

      name = mkOption {
        type = types.passwdEntry types.str;
        description = ''
          The name of the group. If undefined, the name of the attribute set
          will be used.
        '';
      };

      gid = mkOption {
        type = with types; nullOr int;
        default = null;
        description = ''
          The group GID. If the GID is null, a free GID is picked on
          activation.
        '';
      };

      members = mkOption {
        type = with types; listOf (passwdEntry str);
        default = [];
        description = ''
          The user names of the group members, added to the
          `/etc/group` file.
        '';
      };

    };

    config = {
      name = mkDefault name;

      members = mapAttrsToList (n: u: u.name) (
        filterAttrs (n: u: elem config.name u.extraGroups) cfg.users
      );
    };

  };

  subordinateUidRange = {
    options = {
      startUid = mkOption {
        type = types.int;
        description = ''
          Start of the range of subordinate user ids that user is
          allowed to use.
        '';
      };
      count = mkOption {
        type = types.int;
        default = 1;
        description = "Count of subordinate user ids";
      };
    };
  };

  subordinateGidRange = {
    options = {
      startGid = mkOption {
        type = types.int;
        description = ''
          Start of the range of subordinate group ids that user is
          allowed to use.
        '';
      };
      count = mkOption {
        type = types.int;
        default = 1;
        description = "Count of subordinate group ids";
      };
    };
  };

  idsAreUnique = set: idAttr: !(foldr (name: args@{ dup, acc }:
    let
      id = toString (getAttr idAttr (getAttr name set));
      exists = hasAttr id acc;
      newAcc = acc // (listToAttrs [ { name = id; value = true; } ]);
    in if dup then args else if exists
      then trace "Duplicate ${idAttr} ${id}" { dup = true; acc = null; }
      else { dup = false; acc = newAcc; }
    ) { dup = false; acc = {}; } (attrNames set)).dup;

  uidsAreUnique = idsAreUnique (filterAttrs (n: u: u.uid != null) cfg.users) "uid";
  gidsAreUnique = idsAreUnique (filterAttrs (n: g: g.gid != null) cfg.groups) "gid";
  sdInitrdUidsAreUnique = idsAreUnique (filterAttrs (n: u: u.uid != null) config.boot.initrd.systemd.users) "uid";
  sdInitrdGidsAreUnique = idsAreUnique (filterAttrs (n: g: g.gid != null) config.boot.initrd.systemd.groups) "gid";
  groupNames = lib.mapAttrsToList (n: g: g.name) cfg.groups;
  usersWithoutExistingGroup = lib.filterAttrs (n: u: u.group != "" && !lib.elem u.group groupNames) cfg.users;

  spec = pkgs.writeText "users-groups.json" (builtins.toJSON {
    inherit (cfg) mutableUsers;
    users = mapAttrsToList (_: u:
      { inherit (u)
          name uid group description home homeMode createHome isSystemUser
          password hashedPasswordFile hashedPassword
          autoSubUidGidRange subUidRanges subGidRanges
          initialPassword initialHashedPassword expires;
        shell = utils.toShellPath u.shell;
      }) cfg.users;
    groups = attrValues cfg.groups;
  });

  systemShells =
    let
      shells = mapAttrsToList (_: u: u.shell) cfg.users;
    in
      filter types.shellPackage.check shells;

  lingeringUsers = map (u: u.name) (attrValues (flip filterAttrs cfg.users (n: u: u.linger)));
in {
  imports = [
    (mkAliasOptionModuleMD [ "users" "extraUsers" ] [ "users" "users" ])
    (mkAliasOptionModuleMD [ "users" "extraGroups" ] [ "users" "groups" ])
    (mkRenamedOptionModule ["security" "initialRootPassword"] ["users" "users" "root" "initialHashedPassword"])
  ];

  ###### interface
  options = {

    users.mutableUsers = mkOption {
      type = types.bool;
      default = true;
      description = ''
        If set to `true`, you are free to add new users and groups to the system
        with the ordinary `useradd` and
        `groupadd` commands. On system activation, the
        existing contents of the `/etc/passwd` and
        `/etc/group` files will be merged with the
        contents generated from the `users.users` and
        `users.groups` options.
        The initial password for a user will be set
        according to `users.users`, but existing passwords
        will not be changed.

        ::: {.warning}
        If set to `false`, the contents of the user and
        group files will simply be replaced on system activation. This also
        holds for the user passwords; all changed
        passwords will be reset according to the
        `users.users` configuration on activation.
        :::
      '';
    };

    users.enforceIdUniqueness = mkOption {
      type = types.bool;
      default = true;
      description = ''
        Whether to require that no two users/groups share the same uid/gid.
      '';
    };

    users.users = mkOption {
      default = {};
      type = with types; attrsOf (submodule userOpts);
      example = {
        alice = {
          uid = 1234;
          description = "Alice Q. User";
          home = "/home/alice";
          createHome = true;
          group = "users";
          extraGroups = ["wheel"];
          shell = "/bin/sh";
        };
      };
      description = ''
        Additional user accounts to be created automatically by the system.
        This can also be used to set options for root.
      '';
    };

    users.groups = mkOption {
      default = {};
      example =
        { students.gid = 1001;
          hackers = { };
        };
      type = with types; attrsOf (submodule groupOpts);
      description = ''
        Additional groups to be created automatically by the system.
      '';
    };


    users.allowNoPasswordLogin = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Disable checking that at least the `root` user or a user in the `wheel` group can log in using
        a password or an SSH key.

        WARNING: enabling this can lock you out of your system. Enable this only if you know what are you doing.
      '';
    };

    # systemd initrd
    boot.initrd.systemd.users = mkOption {
      description = ''
        Users to include in initrd.
      '';
      default = {};
      type = types.attrsOf (types.submodule ({ name, ... }: {
        options.uid = mkOption {
          type = types.int;
          description = ''
            ID of the user in initrd.
          '';
          defaultText = literalExpression "config.users.users.\${name}.uid";
          default = cfg.users.${name}.uid;
        };
        options.group = mkOption {
          type = types.singleLineStr;
          description = ''
            Group the user belongs to in initrd.
          '';
          defaultText = literalExpression "config.users.users.\${name}.group";
          default = cfg.users.${name}.group;
        };
        options.shell = mkOption {
          type = types.passwdEntry types.path;
          description = ''
            The path to the user's shell in initrd.
          '';
          default = "${pkgs.shadow}/bin/nologin";
          defaultText = literalExpression "\${pkgs.shadow}/bin/nologin";
        };
      }));
    };

    boot.initrd.systemd.groups = mkOption {
      description = ''
        Groups to include in initrd.
      '';
      default = {};
      type = types.attrsOf (types.submodule ({ name, ... }: {
        options.gid = mkOption {
          type = types.int;
          description = ''
            ID of the group in initrd.
          '';
          defaultText = literalExpression "config.users.groups.\${name}.gid";
          default = cfg.groups.${name}.gid;
        };
      }));
    };
  };


  ###### implementation

  config = let
    cryptSchemeIdPatternGroup = "(${lib.concatStringsSep "|" pkgs.libxcrypt.enabledCryptSchemeIds})";
  in {

    users.users = {
      root = {
        uid = ids.uids.root;
        description = "System administrator";
        home = "/root";
        shell = mkDefault cfg.defaultUserShell;
        group = "root";
      };
      nobody = {
        uid = ids.uids.nobody;
        isSystemUser = true;
        description = "Unprivileged account (don't use!)";
        group = "nogroup";
      };
    };

    users.groups = {
      root.gid = ids.gids.root;
      wheel.gid = ids.gids.wheel;
      disk.gid = ids.gids.disk;
      kmem.gid = ids.gids.kmem;
      tty.gid = ids.gids.tty;
      floppy.gid = ids.gids.floppy;
      uucp.gid = ids.gids.uucp;
      lp.gid = ids.gids.lp;
      cdrom.gid = ids.gids.cdrom;
      tape.gid = ids.gids.tape;
      audio.gid = ids.gids.audio;
      video.gid = ids.gids.video;
      dialout.gid = ids.gids.dialout;
      nogroup.gid = ids.gids.nogroup;
      users.gid = ids.gids.users;
      nixbld.gid = ids.gids.nixbld;
      utmp.gid = ids.gids.utmp;
      adm.gid = ids.gids.adm;
      input.gid = ids.gids.input;
      kvm.gid = ids.gids.kvm;
      render.gid = ids.gids.render;
      sgx.gid = ids.gids.sgx;
      shadow.gid = ids.gids.shadow;
    };

    system.activationScripts.users = if !config.systemd.sysusers.enable then {
      supportsDryActivation = true;
      text = ''
        install -m 0700 -d /root
        install -m 0755 -d /home

        ${pkgs.perl.withPackages (p: [ p.FileSlurp p.JSON ])}/bin/perl \
        -w ${./update-users-groups.pl} ${spec}
      '';
    } else ""; # keep around for backwards compatibility

    systemd.services.linger-users = lib.mkIf ((length lingeringUsers) > 0) {
      wantedBy = ["multi-user.target"];
      after = ["systemd-logind.service"];
      requires = ["systemd-logind.service"];

      script = let
        lingerDir = "/var/lib/systemd/linger";
        lingeringUsersFile = builtins.toFile "lingering-users"
          (concatStrings (map (s: "${s}\n")
            (sort (a: b: a < b) lingeringUsers)));  # this sorting is important for `comm` to work correctly
      in ''
        mkdir -vp ${lingerDir}
        cd ${lingerDir}
        for user in $(ls); do
          if ! id "$user" >/dev/null; then
            echo "Removing linger for missing user $user"
            rm --force -- "$user"
          fi
        done
        ls | sort | comm -3 -1 ${lingeringUsersFile} - | xargs -r ${pkgs.systemd}/bin/loginctl disable-linger
        ls | sort | comm -3 -2 ${lingeringUsersFile} - | xargs -r ${pkgs.systemd}/bin/loginctl  enable-linger
      '';

      serviceConfig.Type = "oneshot";
    };

    # Warn about user accounts with deprecated password hashing schemes
    # This does not work when the users and groups are created by
    # systemd-sysusers because the users are created too late then.
    system.activationScripts.hashes = if !config.systemd.sysusers.enable then {
      deps = [ "users" ];
      text = ''
        users=()
        while IFS=: read -r user hash _; do
          if [[ "$hash" = "$"* && ! "$hash" =~ ^\''$${cryptSchemeIdPatternGroup}\$ ]]; then
            users+=("$user")
          fi
        done </etc/shadow

        if (( "''${#users[@]}" )); then
          echo "
        WARNING: The following user accounts rely on password hashing algorithms
        that have been removed. They need to be renewed as soon as possible, as
        they do prevent their users from logging in."
          printf ' - %s\n' "''${users[@]}"
        fi
      '';
    } else ""; # keep around for backwards compatibility

    # for backwards compatibility
    system.activationScripts.groups = stringAfter [ "users" ] "";

    # Install all the user shells
    environment.systemPackages = systemShells;

    environment.etc = mapAttrs' (_: { packages, name, ... }: {
      name = "profiles/per-user/${name}";
      value.source = pkgs.buildEnv {
        name = "user-environment";
        paths = packages;
        inherit (config.environment) pathsToLink extraOutputsToInstall;
        inherit (config.system.path) ignoreCollisions postBuild;
      };
    }) (filterAttrs (_: u: u.packages != []) cfg.users);

    environment.profiles = [
      "$HOME/.nix-profile"
      "\${XDG_STATE_HOME}/nix/profile"
      "$HOME/.local/state/nix/profile"
      "/etc/profiles/per-user/$USER"
    ];

    # systemd initrd
    boot.initrd.systemd = lib.mkIf config.boot.initrd.systemd.enable {
      contents = {
        "/etc/passwd".text = ''
          ${lib.concatStringsSep "\n" (lib.mapAttrsToList (n: { uid, group, shell }: let
            g = config.boot.initrd.systemd.groups.${group};
          in "${n}:x:${toString uid}:${toString g.gid}::/var/empty:${shell}") config.boot.initrd.systemd.users)}
        '';
        "/etc/group".text = ''
          ${lib.concatStringsSep "\n" (lib.mapAttrsToList (n: { gid }: "${n}:x:${toString gid}:") config.boot.initrd.systemd.groups)}
        '';
        "/etc/shells".text = lib.concatStringsSep "\n" (lib.unique (lib.mapAttrsToList (_: u: u.shell) config.boot.initrd.systemd.users)) + "\n";
      };

      storePaths = [ "${pkgs.shadow}/bin/nologin" ];

      users = {
        root = { shell = lib.mkDefault "/bin/bash"; };
        nobody = {};
      };

      groups = {
        root = {};
        nogroup = {};
        systemd-journal = {};
        tty = {};
        dialout = {};
        kmem = {};
        input = {};
        video = {};
        render = {};
        sgx = {};
        audio = {};
        video = {};
        lp = {};
        disk = {};
        cdrom = {};
        tape = {};
        kvm = {};
      };
    };

    assertions = [
      { assertion = !cfg.enforceIdUniqueness || (uidsAreUnique && gidsAreUnique);
        message = "UIDs and GIDs must be unique!";
      }
      { assertion = !cfg.enforceIdUniqueness || (sdInitrdUidsAreUnique && sdInitrdGidsAreUnique);
        message = "systemd initrd UIDs and GIDs must be unique!";
      }
      { assertion = usersWithoutExistingGroup == {};
        message =
          let
            errUsers = lib.attrNames usersWithoutExistingGroup;
            missingGroups = lib.unique (lib.mapAttrsToList (n: u: u.group) usersWithoutExistingGroup);
            mkConfigHint = group: "users.groups.${group} = {};";
          in ''
            The following users have a primary group that is undefined: ${lib.concatStringsSep " " errUsers}
            Hint: Add this to your NixOS configuration:
              ${lib.concatStringsSep "\n  " (map mkConfigHint missingGroups)}
          '';
      }
      { # If mutableUsers is false, to prevent users creating a
        # configuration that locks them out of the system, ensure that
        # there is at least one "privileged" account that has a
        # password or an SSH authorized key. Privileged accounts are
        # root and users in the wheel group.
        # The check does not apply when users.disableLoginPossibilityAssertion
        # The check does not apply when users.mutableUsers
        assertion = !cfg.mutableUsers -> !cfg.allowNoPasswordLogin ->
          any id (mapAttrsToList (name: cfg:
            (name == "root"
             || cfg.group == "wheel"
             || elem "wheel" cfg.extraGroups)
            &&
            (allowsLogin cfg.hashedPassword
             || cfg.password != null
             || cfg.hashedPasswordFile != null
             || cfg.openssh.authorizedKeys.keys != []
             || cfg.openssh.authorizedKeys.keyFiles != [])
          ) cfg.users ++ [
            config.security.googleOsLogin.enable
          ]);
        message = ''
          Neither the root account nor any wheel user has a password or SSH authorized key.
          You must set one to prevent being locked out of your system.
          If you really want to be locked out of your system, set users.allowNoPasswordLogin = true;
          However you are most probably better off by setting users.mutableUsers = true; and
          manually running passwd root to set the root password.
          '';
      }
    ] ++ flatten (flip mapAttrsToList cfg.users (name: user:
      [
        {
        assertion = (user.hashedPassword != null)
        -> (match ".*:.*" user.hashedPassword == null);
        message = ''
            The password hash of user "${user.name}" contains a ":" character.
            This is invalid and would break the login system because the fields
            of /etc/shadow (file where hashes are stored) are colon-separated.
            Please check the value of option `users.users."${user.name}".hashedPassword`.'';
          }
          {
            assertion = let
              isEffectivelySystemUser = user.isSystemUser || (user.uid != null && user.uid < 1000);
            in xor isEffectivelySystemUser user.isNormalUser;
            message = ''
              Exactly one of users.users.${user.name}.isSystemUser and users.users.${user.name}.isNormalUser must be set.
            '';
          }
          {
            assertion = user.group != "";
            message = ''
              users.users.${user.name}.group is unset. This used to default to
              nogroup, but this is unsafe. For example you can create a group
              for this user with:
              users.users.${user.name}.group = "${user.name}";
              users.groups.${user.name} = {};
            '';
          }
        ] ++ (map (shell: {
            assertion = !user.ignoreShellProgramCheck -> (user.shell == pkgs.${shell}) -> (config.programs.${shell}.enable == true);
            message = ''
              users.users.${user.name}.shell is set to ${shell}, but
              programs.${shell}.enable is not true. This will cause the ${shell}
              shell to lack the basic nix directories in its PATH and might make
              logging in as that user impossible. You can fix it with:
              programs.${shell}.enable = true;

              If you know what you're doing and you are fine with the behavior,
              set users.users.${user.name}.ignoreShellProgramCheck = true;
              instead.
            '';
          }) [
          "fish"
          "xonsh"
          "zsh"
        ])
    ));

    warnings =
      flip concatMap (attrValues cfg.users) (user: let
        unambiguousPasswordConfiguration = 1 >= length (filter (x: x != null) ([
          user.hashedPassword
          user.hashedPasswordFile
          user.password
        ] ++ optionals cfg.mutableUsers [
          # For immutable users, initialHashedPassword is set to hashedPassword,
          # so using these options would always trigger the assertion.
          user.initialHashedPassword
          user.initialPassword
        ]));
      in optional (!unambiguousPasswordConfiguration) ''
        The user '${user.name}' has multiple of the options
        `hashedPassword`, `password`, `hashedPasswordFile`, `initialPassword`
        & `initialHashedPassword` set to a non-null value.
        The options silently discard others by the order of precedence
        given above which can lead to surprising results. To resolve this warning,
        set at most one of the options above to a non-`null` value.
      '')
      ++ filter (x: x != null) (
        flip mapAttrsToList cfg.users (_: user:
        # This regex matches a subset of the Modular Crypto Format (MCF)[1]
        # informal standard. Since this depends largely on the OS or the
        # specific implementation of crypt(3) we only support the (sane)
        # schemes implemented by glibc and BSDs. In particular the original
        # DES hash is excluded since, having no structure, it would validate
        # common mistakes like typing the plaintext password.
        #
        # [1]: https://en.wikipedia.org/wiki/Crypt_(C)
        let
          sep = "\\$";
          base64 = "[a-zA-Z0-9./]+";
          id = cryptSchemeIdPatternGroup;
          name = "[a-z0-9-]+";
          value = "[a-zA-Z0-9/+.-]+";
          options = "${name}(=${value})?(,${name}=${value})*";
          scheme  = "${id}(${sep}${options})?";
          content = "${base64}${sep}${base64}(${sep}${base64})?";
          mcf = "^${sep}${scheme}${sep}${content}$";
        in
        if (allowsLogin user.hashedPassword
            && user.hashedPassword != ""  # login without password
            && match mcf user.hashedPassword == null)
        then ''
          The password hash of user "${user.name}" may be invalid. You must set a
          valid hash or the user will be locked out of their account. Please
          check the value of option `users.users."${user.name}".hashedPassword`.''
        else null)
        ++ flip mapAttrsToList cfg.users (name: user:
          if user.passwordFile != null then
            ''The option `users.users."${name}".passwordFile' has been renamed '' +
            ''to `users.users."${name}".hashedPasswordFile'.''
          else null)
      );
  };

}
