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

with lib;

let

  inInitrd = any (fs: fs == "btrfs") config.boot.initrd.supportedFilesystems;
  inSystem = any (fs: fs == "btrfs") config.boot.supportedFilesystems;

  cfgScrub = config.services.btrfs.autoScrub;

  enableAutoScrub = cfgScrub.enable;
  enableBtrfs = inInitrd || inSystem || enableAutoScrub;

in

{
  options = {
    # One could also do regular btrfs balances, but that shouldn't be necessary
    # during normal usage and as long as the filesystems aren't filled near capacity
    services.btrfs.autoScrub = {
      enable = mkEnableOption "regular btrfs scrub";

      fileSystems = mkOption {
        type = types.listOf types.path;
        example = [ "/" ];
        description = ''
          List of paths to btrfs filesystems to regularily call <command>btrfs scrub</command> on.
          Defaults to all mount points with btrfs filesystems.
          If you mount a filesystem multiple times or additionally mount subvolumes,
          you need to manually specify this list to avoid scrubbing multiple times.
        '';
      };

      interval = mkOption {
        default = "monthly";
        type = types.str;
        example = "weekly";
        description = ''
          Systemd calendar expression for when to scrub btrfs filesystems.
          The recommended period is a month but could be less
          (<citerefentry><refentrytitle>btrfs-scrub</refentrytitle>
          <manvolnum>8</manvolnum></citerefentry>).
          See
          <citerefentry><refentrytitle>systemd.time</refentrytitle>
          <manvolnum>7</manvolnum></citerefentry>
          for more information on the syntax.
        '';
      };

    };
  };

  config = mkMerge [
    (mkIf enableBtrfs {
      system.fsPackages = [ pkgs.btrfs-progs ];

      boot.initrd.kernelModules = mkIf inInitrd [ "btrfs" "crc32c" ];

      boot.initrd.extraUtilsCommands = mkIf inInitrd
      ''
        copy_bin_and_libs ${pkgs.btrfs-progs}/bin/btrfs
        ln -sv btrfs $out/bin/btrfsck
        ln -sv btrfsck $out/bin/fsck.btrfs
      '';

      boot.initrd.extraUtilsCommandsTest = mkIf inInitrd
      ''
        $out/bin/btrfs --version
      '';

      boot.initrd.postDeviceCommands = mkIf inInitrd
      ''
        btrfs device scan
      '';
    })

    (mkIf enableAutoScrub {
      assertions = [
        {
          assertion = cfgScrub.enable -> (cfgScrub.fileSystems != []);
          message = ''
            If 'services.btrfs.autoScrub' is enabled, you need to have at least one
            btrfs file system mounted via 'fileSystems' or specify a list manually
            in 'services.btrfs.autoScrub.fileSystems'.
          '';
        }
      ];

      # This will yield duplicated units if the user mounts a filesystem multiple times
      # or additionally mounts subvolumes, but going the other way around via devices would
      # yield duplicated units when a filesystem spans multiple devices.
      # This way around seems like the more sensible default.
      services.btrfs.autoScrub.fileSystems = mkDefault (mapAttrsToList (name: fs: fs.mountPoint)
      (filterAttrs (name: fs: fs.fsType == "btrfs") config.fileSystems));

      # TODO: Did not manage to do it via the usual btrfs-scrub@.timer/.service
      # template units due to problems enabling the parameterized units,
      # so settled with many units and templating via nix for now.
      # https://github.com/NixOS/nixpkgs/pull/32496#discussion_r156527544
      systemd.timers = let
        scrubTimer = fs: let
          fs' = utils.escapeSystemdPath fs;
        in nameValuePair "btrfs-scrub-${fs'}" {
          description = "regular btrfs scrub timer on ${fs}";

          wantedBy = [ "timers.target" ];
          timerConfig = {
            OnCalendar = cfgScrub.interval;
            AccuracySec = "1d";
            Persistent = true;
          };
        };
      in listToAttrs (map scrubTimer cfgScrub.fileSystems);

      systemd.services = let
        scrubService = fs: let
          fs' = utils.escapeSystemdPath fs;
        in nameValuePair "btrfs-scrub-${fs'}" {
          description = "btrfs scrub on ${fs}";
          # scrub prevents suspend2ram or proper shutdown
          conflicts = [ "shutdown.target" "sleep.target" ];
          before = [ "shutdown.target" "sleep.target" ];

          serviceConfig = {
            # simple and not oneshot, otherwise ExecStop is not used
            Type = "simple";
            Nice = 19;
            IOSchedulingClass = "idle";
            ExecStart = "${pkgs.btrfs-progs}/bin/btrfs scrub start -B ${fs}";
            ExecStop  = "${pkgs.btrfs-progs}/bin/btrfs scrub cancel ${fs}";
          };
        };
      in listToAttrs (map scrubService cfgScrub.fileSystems);
    })
  ];
}