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

with lib;

let
  cfg = config.services.tarsnap;

  optionalNullStr = e: v: if e == null then "" else v;

  configFile = cfg: ''
    cachedir ${config.services.tarsnap.cachedir}
    keyfile  ${config.services.tarsnap.keyfile}
    ${optionalString cfg.nodump "nodump"}
    ${optionalString cfg.printStats "print-stats"}
    ${optionalString cfg.printStats "humanize-numbers"}
    ${optionalNullStr cfg.checkpointBytes "checkpoint-bytes "+cfg.checkpointBytes}
    ${optionalString cfg.aggressiveNetworking "aggressive-networking"}
    ${concatStringsSep "\n" (map (v: "exclude "+v) cfg.excludes)}
    ${concatStringsSep "\n" (map (v: "include "+v) cfg.includes)}
    ${optionalString cfg.lowmem "lowmem"}
    ${optionalString cfg.verylowmem "verylowmem"}
  '';
in
{
  options = {
    services.tarsnap = {
      enable = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Enable periodic tarsnap backups.
        '';
      };

      keyfile = mkOption {
        type = types.str;
        default = "/root/tarsnap.key";
        description = ''
          The keyfile which associates this machine with your tarsnap
          account.
          Create the keyfile with <command>tarsnap-keygen</command>.

          The keyfile name should be given as a string and not a path, to
          avoid the key being copied into the Nix store.
        '';
      };

      cachedir = mkOption {
        type    = types.nullOr types.path;
        default = "/var/cache/tarsnap";
        description = ''
          The cache allows tarsnap to identify previously stored data
          blocks, reducing archival time and bandwidth usage.

          Should the cache become desynchronized or corrupted, tarsnap
          will refuse to run until you manually rebuild the cache with
          <command>tarsnap --fsck</command>.

          Set to <literal>null</literal> to disable caching.
        '';
      };

      archives = mkOption {
        type = types.attrsOf (types.submodule (
          {
            options = {
              nodump = mkOption {
                type = types.bool;
                default = true;
                description = ''
                  Exclude files with the <literal>nodump</literal> flag.
                '';
              };

              printStats = mkOption {
                type = types.bool;
                default = true;
                description = ''
                  Print global archive statistics upon completion.
                  The output is available via
                  <command>systemctl status tarsnap@archive-name</command>.
                '';
              };

              checkpointBytes = mkOption {
                type = types.nullOr types.str;
                default = "1GB";
                description = ''
                  Create a checkpoint every <literal>checkpointBytes</literal>
                  of uploaded data (optionally specified using an SI prefix).

                  1GB is the minimum value. A higher value is recommended,
                  as checkpointing is expensive.

                  Set to <literal>null</literal> to disable checkpointing.
                '';
              };

              period = mkOption {
                type = types.str;
                default = "01:15";
                example = "hourly";
                description = ''
                  Create archive at this interval.

                  The format is described in
                  <citerefentry><refentrytitle>systemd.time</refentrytitle>
                  <manvolnum>7</manvolnum></citerefentry>.
                '';
              };

              aggressiveNetworking = mkOption {
                type = types.bool;
                default = false;
                description = ''
                  Upload data over multiple TCP connections, potentially
                  increasing tarsnap's bandwidth utilisation at the cost
                  of slowing down all other network traffic. Not
                  recommended unless TCP congestion is the dominant
                  limiting factor.
                '';
              };

              directories = mkOption {
                type = types.listOf types.path;
                default = [];
                description = "List of filesystem paths to archive.";
              };

              excludes = mkOption {
                type = types.listOf types.str;
                default = [];
                description = ''
                  Exclude files and directories matching these patterns.
                '';
              };

              includes = mkOption {
                type = types.listOf types.str;
                default = [];
                description = ''
                  Include only files and directories matching these
                  patterns (the empty list includes everything).

                  Exclusions have precedence over inclusions.
                '';
              };

              lowmem = mkOption {
                type = types.bool;
                default = false;
                description = ''
                  Reduce memory consumption by not caching small files.
                  Possibly beneficial if the average file size is smaller
                  than 1 MB and the number of files is lower than the
                  total amount of RAM in KB.
                '';
              };

              verylowmem = mkOption {
                type = types.bool;
                default = false;
                description = ''
                  Reduce memory consumption by a factor of 2 beyond what
                  <literal>lowmem</literal> does, at the cost of significantly
                  slowing down the archiving process.
                '';
              };
            };
          }
        ));

        default = {};

        example = literalExample ''
          {
            nixos =
              { directories = [ "/home" "/root/ssl" ];
              };

            gamedata =
              { directories = [ "/var/lib/minecraft "];
                period      = "*:30";
              };
          }
        '';

        description = ''
          Tarsnap archive configurations. Each attribute names an archive
          to be created at a given time interval, according to the options
          associated with it. When uploading to the tarsnap server,
          archive names are suffixed by a 1 second resolution timestamp.

          For each member of the set is created a timer which triggers the
          instanced <literal>tarsnap@</literal> service unit. You may use
          <command>systemctl start tarsnap@archive-name</command> to
          manually trigger creation of <literal>archive-name</literal> at
          any time.
        '';
      };
    };
  };

  config = mkIf cfg.enable {
    assertions =
      (mapAttrsToList (name: cfg:
        { assertion = cfg.directories != [];
          message = "Must specify paths for tarsnap to back up";
        }) cfg.archives) ++
      (mapAttrsToList (name: cfg:
        { assertion = !(cfg.lowmem && cfg.verylowmem);
          message = "You cannot set both lowmem and verylowmem";
        }) cfg.archives);

    systemd.services."tarsnap@" = {
      description = "Tarsnap archive '%i'";
      requires    = [ "network.target" ];

      path = [ pkgs.tarsnap pkgs.coreutils ];
      scriptArgs = "%i";
      script = ''
        mkdir -p -m 0755 ${dirOf cfg.cachedir}
        mkdir -p -m 0700 ${cfg.cachedir}
        DIRS=`cat /etc/tarsnap/$1.dirs`
        exec tarsnap --configfile /etc/tarsnap/$1.conf -c -f $1-$(date +"%Y%m%d%H%M%S") $DIRS
      '';

      serviceConfig = {
        IOSchedulingClass = "idle";
        NoNewPrivileges = "true";
        CapabilityBoundingSet = "CAP_DAC_READ_SEARCH";
      };
    };

    systemd.timers = mapAttrs' (name: cfg: nameValuePair "tarsnap@${name}"
      { timerConfig.OnCalendar = cfg.period;
        wantedBy = [ "timers.target" ];
      }) cfg.archives;

    environment.etc =
      (mapAttrs' (name: cfg: nameValuePair "tarsnap/${name}.conf"
        { text = configFile cfg;
        }) cfg.archives) //
      (mapAttrs' (name: cfg: nameValuePair "tarsnap/${name}.dirs"
        { text = concatStringsSep " " cfg.directories;
        }) cfg.archives);

    environment.systemPackages = [ pkgs.tarsnap ];
  };
}