{pkgs, config, ...}:

###### interface
let
  inherit (pkgs.lib) mkOption mkIf;

  options = {
    services = {
      postfix = {
        enable = mkOption {
          default = false;
          description ="
            Whether to run the Postfix mail server.
          ";
        };
        user = mkOption {
          default = "postfix";
          description = "
            How to call postfix user (must be used only for postfix).
          ";
        };
        group = mkOption {
          default = "postfix";
          description = "
            How to call postfix group (must be used only for postfix).
          ";
        };
        setgidGroup = mkOption {
          default = "postdrop";
          description = "
            How to call postfix setgid group (for postdrop). Should 
            be uniquely used group.
          ";
        };
        networks = mkOption {
          default = null;
          example = ["192.168.0.1/24"];
          description = "
            Net masks for trusted - allowed to relay mail to third parties - 
            hosts. Leave empty to use mynetworks_style configuration or use 
            default (localhost-only).
          ";
        };
        networksStyle = mkOption {
          default = "";
          description = "
            Name of standard way of trusted network specification to use,
            leave blank if you specify it explicitly or if you want to use 
            default (localhost-only).
          ";
        };
        hostname = mkOption {
          default = "";
          description ="
            Hostname to use. Leave blank to use just the hostname of machine.
            It should be FQDN.
          ";
        };
        domain = mkOption {
          default = "";
          description ="
            Domain to use. Leave blank to use hostname minus first component.
          ";
        };
        origin = mkOption {
          default = "";
          description ="
            Origin to use in outgoing e-mail. Leave blank to use hostname.
          ";
        };
        destination = mkOption {
          default = null;
          example = ["localhost"];
          description = "
            Full (!) list of domains we deliver locally. Leave blank for 
            acceptable Postfix default.
          ";
        };
        relayDomains = mkOption {
          default = null;
          example = ["localdomain"];
          description = "
            List of domains we agree to relay to. Default is the same as 
            destination.
          ";
        };
        relayHost = mkOption {
          default = "";
          description = "
            Mail relay for outbound mail.
          ";
        };
        lookupMX = mkOption {
          default = false;
          description = "
            Whether relay specified is just domain whose MX must be used.
          ";
        };
        postmasterAlias = mkOption {
          default = "root";
          description = "
            Who should receive postmaster e-mail.
          ";
        };
        rootAlias = mkOption {
          default = "";
          description = "
            Who should receive root e-mail. Blank for no redirection.
          ";
        };
        extraAliases = mkOption {
          default = "";
          description = "
            Additional entries to put verbatim into aliases file.
          ";
        };

        sslCert = mkOption {
          default = "";
          description = "
            SSL certificate to use.
          ";
        };
        sslCACert = mkOption {
          default = "";
          description = "
            SSL certificate of CA.
          ";
        };
        sslKey = mkOption {
          default = "";
          description ="
            SSL key to use.
          ";
        };

        recipientDelimiter = mkOption {
          default = "";
          example = "+";
          description = "
            Delimiter for address extension: so mail to user+test can be handled by ~user/.forward+test
          ";
        };

      };
    };
  };
in

###### implementation

let
  startingDependency = if config.services.gw6c.enable then "gw6c" else "network-interfaces";

  cfg = config.services.postfix;
  user = cfg.user;
  group = cfg.group;
  setgidGroup = cfg.setgidGroup;
  idList = import ../system/ids.nix;

  optionalString = pkgs.lib.optionalString;
  concatStringsSep = pkgs.lib.concatStringsSep;
  mainCf = 
  ''
    queue_directory = /var/postfix/queue
    command_directory = ${pkgs.postfix}/sbin
    daemon_directory = ${pkgs.postfix}/libexec/postfix

    mail_owner = ${user}
    default_privs = nobody

  ''
  + optionalString (config.services.gw6c.enable || config.networking.nativeIPv6) (''
    inet_protocols = all
  '')
  +
  (if cfg.networks!=null then
    (''
      mynetworks = ${concatStringsSep ", " cfg.networks}
    '') 
  else if (cfg.networksStyle != "") then
    (''
      mynetworks_style = ${cfg.networksStyle} 
    '')
  else
    # Postfix default is subnet, but let's play safe
    (''
      mynetworks_style = host
    '')
  )
  + optionalString (cfg.hostname != "") (''
    myhostname = ${cfg.hostname}
  '')
  + optionalString (cfg.domain != "") (''
    mydomain = ${cfg.domain}
  '')
  + optionalString (cfg.origin != "") (''
    myorigin = ${cfg.origin}
  '')
  + optionalString (cfg.destination != null) (''
    mydestination = ${concatStringsSep ", " cfg.destination}
  '')
  + optionalString (cfg.relayDomains != null) (''
    relay_domains = ${concatStringsSep ", " cfg.relayDomains}
  '')
  + ''
    local_recipient_maps = 
  ''
  + (''
    relayhost = ${if cfg.lookupMX || cfg.relayHost == "" then 
        cfg.relayHost 
      else 
        "[" + cfg.relayHost + "]"}
  '')
  + (''
    alias_maps = hash:/var/postfix/conf/aliases

    mail_spool_directory = /var/spool/mail/

    setgid_group = ${setgidGroup}
  '')
  + optionalString (cfg.sslCert != "") (''

    smtp_tls_CAfile = ${cfg.sslCACert}
    smtp_tls_cert_file = ${cfg.sslCert}
    smtp_tls_key_file = ${cfg.sslKey}

    smtp_use_tls = yes

    smtpd_tls_CAfile = ${cfg.sslCACert}
    smtpd_tls_cert_file = ${cfg.sslCert}
    smtpd_tls_key_file = ${cfg.sslKey}

    smtpd_use_tls = yes 

    recipientDelimiter = ${cfg.recipientDelimiter}

  '')
  ;

  aliases = 
    (optionalString (cfg.postmasterAlias != "") (''
      postmaster: ${cfg.postmasterAlias}
    ''))
    +
    (optionalString (cfg.rootAlias != "") (''
      root: ${cfg.rootAlias}
    ''))
    + cfg.extraAliases
  ;

  aliasesFile = pkgs.writeText "postfix-aliases" aliases;
  mainCfFile = pkgs.writeText "postfix-main.cf" mainCf;
 
in

mkIf config.services.postfix.enable {
  require = [
    options
  ];

  environment = {
    etc = [{
      source = "/var/postfix/conf";
      target = "postfix";
    }];
  };

  users = {
    extraUsers = [
      { name = user;
        description = "Postfix mail server user";
        uid = idList.uids.postfix;
        group = group;
      }
    ];

    extraGroups = [
      { name = group; 
        gid = idList.gids.postfix;
      }
      { name = setgidGroup; 
        gid = idList.gids.postdrop;
      }
    ];
  };

  services = {
    extraJobs = [{
      name = "postfix";


      # I copy _lots_ of shipped configuration filed 
      # that can be left as is. I am afraid the exact
      # will list slightly change in next Postfix 
      # release, so listing them all one-by-one in an 
      # accurate way is unlikely to be better.
      job = ''
        description "Postfix mail server job"

        start on ${startingDependency}/started
        stop on never

        script 
            if ! [ -d /var/spool/postfix ]; then
              ${pkgs.coreutils}/bin/mkdir -p /var/spool/mail /var/postfix/conf /var/postfix/queue
            fi
              
            ${pkgs.coreutils}/bin/chown -R ${user}.${group} /var/postfix 
            ${pkgs.coreutils}/bin/chown -R ${user}.${setgidGroup} /var/postfix/queue 
            ${pkgs.coreutils}/bin/chmod -R ug+rwX /var/postfix/queue 
            ${pkgs.coreutils}/bin/chown root.root /var/spool/mail
            ${pkgs.coreutils}/bin/chmod a+rwxt /var/spool/mail
            
            ln -sf ${pkgs.postfix}/share/postfix/conf/* /var/postfix/conf

            ln -sf ${aliasesFile} /var/postfix/conf/aliases
            ln -sf ${mainCfFile} /var/postfix/conf/main.cf

            ${pkgs.postfix}/sbin/postalias -c /var/postfix/conf /var/postfix/conf/aliases
            
            ${pkgs.postfix}/sbin/postfix -c /var/postfix/conf start 
        end script
      '';

    }];
  };
}