{pkgs, config, ...}:

with pkgs.lib;
with pkgs;

###### interface
let
  inherit mkOption mkIf optionalString stringAfter singleton;

  cfg = config.users.ldap;

  options = {
    users = {
      ldap = {

        enable = mkOption {
          default = false;
          description = "
            Whether to enable authentication against an LDAP server.
          ";
        };

        server = mkOption {
          example = "ldap://ldap.example.org/";
          description = "
            The URL of the LDAP server.
          ";
        };

        base = mkOption {
          example = "dc=example,dc=org";
          description = "
            The distinguished name of the search base.
          ";
        };

        useTLS = mkOption {
          default = false;
          description = "
            If enabled, use TLS (encryption) over an LDAP (port 389)
            connection.  The alternative is to specify an LDAPS server (port
            636) in <option>users.ldap.server</option> or to forego
            security.
          ";
        };

        timeLimit = mkOption {
          default = 0;
          type = types.int;
          description = "
            Specifies the time limit (in seconds) to use when performing
            searches. A value of zero (0), which is the default, is to
            wait indefinitely for searches to be completed.
          ";
        };

        daemon = {
          enable = mkOption {
            default = false;
            description = ''
              Whether to let the nslcd daemon (nss-pam-ldapd) handle the
              LDAP lookups for NSS and PAM. This can improve performance,
              and if you need to bind to the LDAP server with a password,
              it increases security, since only the nslcd user needs to
              have access to the bindpw file, not everyone that uses NSS
              and/or PAM. If this option is enabled, a local nscd user is
              created automatically, and the nslcd service is started
              automatically when the network get up.
            '';
          };

          extraConfig = mkOption { 
            default =  ""; 
            type = types.string;
            description = '' 
              Extra configuration options that will be added verbatim at
              the end of the nslcd configuration file (nslcd.conf).
            '' ;
          } ; 
        };

        bind = {
          distinguishedName = mkOption {
            default = "";
            example = "cn=admin,dc=example,dc=com";
            type = types.string;
            description = "
              The distinguished name to bind to the LDAP server with. If this
              is not specified, an anonymous bind will be done.
            ";
          };

          password = mkOption {
            default = "/etc/ldap/bind.password";
            type = types.string;
            description = "
              The path to a file containing the credentials to use when binding
              to the LDAP server (if not binding anonymously).
            ";
          };

          timeLimit = mkOption {
            default = 30;
            type = types.int;
            description = "
              Specifies the time limit (in seconds) to use when connecting
              to the directory server. This is distinct from the time limit
              specified in <literal>users.ldap.timeLimit</literal> and affects
              the initial server connection only.
            ";
          };

          policy = mkOption {
            default = "hard_open";
            type = types.string;
            description = "
              Specifies the policy to use for reconnecting to an unavailable
              LDAP server. The default is <literal>hard_open</literal>, which
              reconnects if opening the connection to the directory server
              failed. By contrast, <literal>hard_init</literal> reconnects if
              initializing the connection failed. Initializing may not
              actually contact the directory server, and it is possible that
              a malformed configuration file will trigger reconnection. If
              <literal>soft</literal> is specified, then
              <literal>nss_ldap</literal> will return immediately on server
              failure. All hard reconnect policies block with exponential
              backoff before retrying.
            ";
          };
        };

        extraConfig = mkOption { 
          default = "" ; 
          type = types.string ;
          description = ''
            Extra configuration options that will be added verbatim at
            the end of the ldap configuration file (ldap.conf).
            If <literal>users.ldap.daemon</literal> is enabled, this
            configuration will not be used. In that case, use
            <literal>users.ldap.daemon.extraConfig</literal> instead.
          '' ;
        };

      };
    };
  };

  # Careful: OpenLDAP seems to be very picky about the indentation of
  # this file.  Directives HAVE to start in the first column!
  ldapConfig = {
    target = "ldap.conf";
    source = writeText "ldap.conf" ''
      uri ${config.users.ldap.server}
      base ${config.users.ldap.base}
      timelimit ${toString config.users.ldap.timeLimit}
      bind_timelimit ${toString config.users.ldap.bind.timeLimit}
      bind_policy ${config.users.ldap.bind.policy}
      ${optionalString config.users.ldap.useTLS ''
        ssl start_tls
        tls_checkpeer no
      ''}
      ${optionalString (config.users.ldap.bind.distinguishedName != "") ''
        binddn ${config.users.ldap.bind.distinguishedName}
      ''}
      ${optionalString (cfg.extraConfig != "") cfg.extraConfig }
    '';
  };

  nslcdConfig = {
    target = "nslcd.conf";
    source = writeText "nslcd.conf" ''
      uid nslcd
      gid nslcd
      uri ${cfg.server}
      base ${cfg.base}
      timelimit ${toString cfg.timeLimit}
      bind_timelimit ${toString cfg.bind.timeLimit}
      ${optionalString (cfg.bind.distinguishedName != "")
        "binddn ${cfg.bind.distinguishedName}" }
      ${optionalString (cfg.daemon.extraConfig != "") cfg.daemon.extraConfig }
    '';
  };

  insertLdapPassword = !config.users.ldap.daemon.enable &&
    config.users.ldap.bind.distinguishedName != "";

in

###### implementation
mkIf cfg.enable {
  require = [
    options
  ];

  environment.etc = if cfg.daemon.enable then [nslcdConfig] else [ldapConfig];

  system.activationScripts = mkIf insertLdapPassword {
    ldap = stringAfter [ "etc" "groups" "users" ] ''
      if test -f "${cfg.bind.password}" ; then
        echo "bindpw "$(cat ${cfg.bind.password})"" | cat ${ldapConfig} - > /etc/ldap.conf.bindpw
        mv -fT /etc/ldap.conf.bindpw /etc/ldap.conf
        chmod 600 /etc/ldap.conf
      fi
    '';
  };

  system.nssModules = singleton (
    if cfg.daemon.enable then nss_pam_ldapd else nss_ldap
  );

  users = mkIf cfg.daemon.enable {
    extraGroups.nslcd = {
      gid = config.ids.gids.nslcd;
    };

    extraUsers.nslcd = {
      uid = config.ids.uids.nslcd;
      description = "nslcd user.";
      group = "nslcd";
    };
  };

  systemd.services = mkIf cfg.daemon.enable {
    nslcd = {
      wantedBy = [ "nss-user-lookup.target" ];

      path = [ nss_pam_ldapd ];
  
      preStart = ''
        mkdir -p /run/nslcd
        rm -f /run/nslcd/nslcd.pid;
        chown nslcd.nslcd /run/nslcd
        ${optionalString (cfg.bind.distinguishedName != "") ''
          if test -s "${cfg.bind.password}" ; then
            ln -sfT "${cfg.bind.password}" /run/nslcd/bindpw
          fi
        ''}
      '';

      script = "nslcd";

      serviceConfig = {
        Type = "forking";
        PIDFile = "/run/nslcd/nslcd.pid";
        Restart = "always";
      };
    };
  };
}