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

with lib;
with utils;

let

  addCheckDesc = desc: elemType: check: types.addCheck elemType check
    // { description = "${elemType.description} (with check: ${desc})"; };

  isNonEmpty = s: (builtins.match "[ \t\n]*" s) == null;
  nonEmptyStr = addCheckDesc "non-empty" types.str isNonEmpty;

  fileSystems' = toposort fsBefore (attrValues config.fileSystems);

  fileSystems = if fileSystems' ? result
                then # use topologically sorted fileSystems everywhere
                     fileSystems'.result
                else # the assertion below will catch this,
                     # but we fall back to the original order
                     # anyway so that other modules could check
                     # their assertions too
                     (attrValues config.fileSystems);

  specialFSTypes = [ "proc" "sysfs" "tmpfs" "ramfs" "devtmpfs" "devpts" ];

  nonEmptyWithoutTrailingSlash = addCheckDesc "non-empty without trailing slash" types.str
    (s: isNonEmpty s && (builtins.match ".+/" s) == null);

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

    options = {
      mountPoint = mkOption {
        example = "/mnt/usb";
        type = nonEmptyWithoutTrailingSlash;
        description = lib.mdDoc "Location of the mounted the file system.";
      };

      device = mkOption {
        default = null;
        example = "/dev/sda";
        type = types.nullOr nonEmptyStr;
        description = lib.mdDoc "Location of the device.";
      };

      fsType = mkOption {
        default = "auto";
        example = "ext3";
        type = nonEmptyStr;
        description = lib.mdDoc "Type of the file system.";
      };

      options = mkOption {
        default = [ "defaults" ];
        example = [ "data=journal" ];
        description = lib.mdDoc "Options used to mount the file system.";
        type = types.listOf nonEmptyStr;
      };

      depends = mkOption {
        default = [ ];
        example = [ "/persist" ];
        type = types.listOf nonEmptyWithoutTrailingSlash;
        description = lib.mdDoc ''
          List of paths that should be mounted before this one. This filesystem's
          {option}`device` and {option}`mountPoint` are always
          checked and do not need to be included explicitly. If a path is added
          to this list, any other filesystem whose mount point is a parent of
          the path will be mounted before this filesystem. The paths do not need
          to actually be the {option}`mountPoint` of some other filesystem.
        '';
      };

    };

    config = {
      mountPoint = mkDefault name;
      device = mkIf (elem config.fsType specialFSTypes) (mkDefault config.fsType);
    };

  };

  fileSystemOpts = { config, ... }: {

    options = {

      label = mkOption {
        default = null;
        example = "root-partition";
        type = types.nullOr nonEmptyStr;
        description = lib.mdDoc "Label of the device (if any).";
      };

      autoFormat = mkOption {
        default = false;
        type = types.bool;
        description = lib.mdDoc ''
          If the device does not currently contain a filesystem (as
          determined by {command}`blkid`, then automatically
          format it with the filesystem type specified in
          {option}`fsType`.  Use with caution.
        '';
      };

      formatOptions = mkOption {
        default = "";
        type = types.str;
        description = lib.mdDoc ''
          If {option}`autoFormat` option is set specifies
          extra options passed to mkfs.
        '';
      };

      autoResize = mkOption {
        default = false;
        type = types.bool;
        description = lib.mdDoc ''
          If set, the filesystem is grown to its maximum size before
          being mounted. (This is typically the size of the containing
          partition.) This is currently only supported for ext2/3/4
          filesystems that are mounted during early boot.
        '';
      };

      noCheck = mkOption {
        default = false;
        type = types.bool;
        description = lib.mdDoc "Disable running fsck on this filesystem.";
      };

    };

    config = let
      defaultFormatOptions =
        # -F needed to allow bare block device without partitions
        if (builtins.substring 0 3 config.fsType) == "ext" then "-F"
        # -q needed for non-interactive operations
        else if config.fsType == "jfs" then "-q"
        # (same here)
        else if config.fsType == "reiserfs" then "-q"
        else null;
    in {
      options = mkIf config.autoResize [ "x-nixos.autoresize" ];
      formatOptions = mkIf (defaultFormatOptions != null) (mkDefault defaultFormatOptions);
    };

  };

  # Makes sequence of `specialMount device mountPoint options fsType` commands.
  # `systemMount` should be defined in the sourcing script.
  makeSpecialMounts = mounts:
    pkgs.writeText "mounts.sh" (concatMapStringsSep "\n" (mount: ''
      specialMount "${mount.device}" "${mount.mountPoint}" "${concatStringsSep "," mount.options}" "${mount.fsType}"
    '') mounts);

  makeFstabEntries =
    let
      fsToSkipCheck = [ "none" "bindfs" "btrfs" "zfs" "tmpfs" "nfs" "vboxsf" "glusterfs" "apfs" "9p" "cifs" "prl_fs" "vmhgfs" ];
      isBindMount = fs: builtins.elem "bind" fs.options;
      skipCheck = fs: fs.noCheck || fs.device == "none" || builtins.elem fs.fsType fsToSkipCheck || isBindMount fs;
      # https://wiki.archlinux.org/index.php/fstab#Filepath_spaces
      escape = string: builtins.replaceStrings [ " " "\t" ] [ "\\040" "\\011" ] string;
    in fstabFileSystems: { rootPrefix ? "", excludeChecks ? false, extraOpts ? (fs: []) }: concatMapStrings (fs:
      (optionalString (isBindMount fs) (escape rootPrefix))
      + (if fs.device != null then escape fs.device
         else if fs.label != null then "/dev/disk/by-label/${escape fs.label}"
         else throw "No device specified for mount point ‘${fs.mountPoint}’.")
      + " " + escape (rootPrefix + fs.mountPoint)
      + " " + fs.fsType
      + " " + builtins.concatStringsSep "," (fs.options ++ (extraOpts fs))
      + " " + (optionalString (!excludeChecks)
        ("0 " + (if skipCheck fs then "0" else if fs.mountPoint == "/" then "1" else "2")))
      + "\n"
    ) fstabFileSystems;

    initrdFstab = pkgs.writeText "initrd-fstab" (makeFstabEntries (filter utils.fsNeededForBoot fileSystems) {
      rootPrefix = "/sysroot";
      excludeChecks = true;
      extraOpts = fs:
        (optional fs.autoResize "x-systemd.growfs")
        ++ (optional fs.autoFormat "x-systemd.makefs");
    });

in

{

  ###### interface

  options = {

    fileSystems = mkOption {
      default = {};
      example = literalExpression ''
        {
          "/".device = "/dev/hda1";
          "/data" = {
            device = "/dev/hda2";
            fsType = "ext3";
            options = [ "data=journal" ];
          };
          "/bigdisk".label = "bigdisk";
        }
      '';
      type = types.attrsOf (types.submodule [coreFileSystemOpts fileSystemOpts]);
      description = lib.mdDoc ''
        The file systems to be mounted.  It must include an entry for
        the root directory (`mountPoint = "/"`).  Each
        entry in the list is an attribute set with the following fields:
        `mountPoint`, `device`,
        `fsType` (a file system type recognised by
        {command}`mount`; defaults to
        `"auto"`), and `options`
        (the mount options passed to {command}`mount` using the
        {option}`-o` flag; defaults to `[ "defaults" ]`).

        Instead of specifying `device`, you can also
        specify a volume label (`label`) for file
        systems that support it, such as ext2/ext3 (see {command}`mke2fs -L`).
      '';
    };

    system.fsPackages = mkOption {
      internal = true;
      default = [ ];
      description = "Packages supplying file system mounters and checkers.";
    };

    boot.supportedFilesystems = mkOption {
      default = [ ];
      example = [ "btrfs" ];
      type = types.listOf types.str;
      description = lib.mdDoc "Names of supported filesystem types.";
    };

    boot.specialFileSystems = mkOption {
      default = {};
      type = types.attrsOf (types.submodule coreFileSystemOpts);
      internal = true;
      description = ''
        Special filesystems that are mounted very early during boot.
      '';
    };

    boot.devSize = mkOption {
      default = "5%";
      example = "32m";
      type = types.str;
      description = lib.mdDoc ''
        Size limit for the /dev tmpfs. Look at mount(8), tmpfs size option,
        for the accepted syntax.
      '';
    };

    boot.devShmSize = mkOption {
      default = "50%";
      example = "256m";
      type = types.str;
      description = lib.mdDoc ''
        Size limit for the /dev/shm tmpfs. Look at mount(8), tmpfs size option,
        for the accepted syntax.
      '';
    };

    boot.runSize = mkOption {
      default = "25%";
      example = "256m";
      type = types.str;
      description = lib.mdDoc ''
        Size limit for the /run tmpfs. Look at mount(8), tmpfs size option,
        for the accepted syntax.
      '';
    };
  };


  ###### implementation

  config = {

    assertions = let
      ls = sep: concatMapStringsSep sep (x: x.mountPoint);
      notAutoResizable = fs: fs.autoResize && !(hasPrefix "ext" fs.fsType || fs.fsType == "f2fs");
    in [
      { assertion = ! (fileSystems' ? cycle);
        message = "The ‘fileSystems’ option can't be topologically sorted: mountpoint dependency path ${ls " -> " fileSystems'.cycle} loops to ${ls ", " fileSystems'.loops}";
      }
      { assertion = ! (any notAutoResizable fileSystems);
        message = let
          fs = head (filter notAutoResizable fileSystems);
        in
          "Mountpoint '${fs.mountPoint}': 'autoResize = true' is not supported for 'fsType = \"${fs.fsType}\"':${if fs.fsType == "auto" then " fsType has to be explicitly set and" else ""} only the ext filesystems and f2fs support it.";
      }
    ];

    # Export for use in other modules
    system.build.fileSystems = fileSystems;
    system.build.earlyMountScript = makeSpecialMounts (toposort fsBefore (attrValues config.boot.specialFileSystems)).result;

    boot.supportedFilesystems = map (fs: fs.fsType) fileSystems;

    # Add the mount helpers to the system path so that `mount' can find them.
    system.fsPackages = [ pkgs.dosfstools ];

    environment.systemPackages = with pkgs; [ fuse3 fuse ] ++ config.system.fsPackages;

    environment.etc.fstab.text =
      let
        swapOptions = sw: concatStringsSep "," (
          sw.options
          ++ optional (sw.priority != null) "pri=${toString sw.priority}"
          ++ optional (sw.discardPolicy != null) "discard${optionalString (sw.discardPolicy != "both") "=${toString sw.discardPolicy}"}"
        );
      in ''
        # This is a generated file.  Do not edit!
        #
        # To make changes, edit the fileSystems and swapDevices NixOS options
        # in your /etc/nixos/configuration.nix file.
        #
        # <file system> <mount point>   <type>  <options>       <dump>  <pass>

        # Filesystems.
        ${makeFstabEntries fileSystems {}}

        # Swap devices.
        ${flip concatMapStrings config.swapDevices (sw:
            "${sw.realDevice} none swap ${swapOptions sw}\n"
        )}
      '';

    boot.initrd.systemd.contents."/etc/fstab".source = initrdFstab;

    # Provide a target that pulls in all filesystems.
    systemd.targets.fs =
      { description = "All File Systems";
        wants = [ "local-fs.target" "remote-fs.target" ];
      };

    systemd.services =

    # Emit systemd services to format requested filesystems.
      let
        formatDevice = fs:
          let
            mountPoint' = "${escapeSystemdPath fs.mountPoint}.mount";
            device'  = escapeSystemdPath fs.device;
            device'' = "${device'}.device";
          in nameValuePair "mkfs-${device'}"
          { description = "Initialisation of Filesystem ${fs.device}";
            wantedBy = [ mountPoint' ];
            before = [ mountPoint' "systemd-fsck@${device'}.service" ];
            requires = [ device'' ];
            after = [ device'' ];
            path = [ pkgs.util-linux ] ++ config.system.fsPackages;
            script =
              ''
                if ! [ -e "${fs.device}" ]; then exit 1; fi
                # FIXME: this is scary.  The test could be more robust.
                type=$(blkid -p -s TYPE -o value "${fs.device}" || true)
                if [ -z "$type" ]; then
                  echo "creating ${fs.fsType} filesystem on ${fs.device}..."
                  mkfs.${fs.fsType} ${fs.formatOptions} "${fs.device}"
                fi
              '';
            unitConfig.RequiresMountsFor = [ "${dirOf fs.device}" ];
            unitConfig.DefaultDependencies = false; # needed to prevent a cycle
            serviceConfig.Type = "oneshot";
          };
      in listToAttrs (map formatDevice (filter (fs: fs.autoFormat && !(utils.fsNeededForBoot fs)) fileSystems)) // {
    # Mount /sys/fs/pstore for evacuating panic logs and crashdumps from persistent storage onto the disk using systemd-pstore.
    # This cannot be done with the other special filesystems because the pstore module (which creates the mount point) is not loaded then.
        "mount-pstore" = {
          serviceConfig = {
            Type = "oneshot";
            # skip on kernels without the pstore module
            ExecCondition = "${pkgs.kmod}/bin/modprobe -b pstore";
            ExecStart = pkgs.writeShellScript "mount-pstore.sh" ''
              set -eu
              # if the pstore module is builtin it will have mounted the persistent store automatically. it may also be already mounted for other reasons.
              ${pkgs.util-linux}/bin/mountpoint -q /sys/fs/pstore || ${pkgs.util-linux}/bin/mount -t pstore -o nosuid,noexec,nodev pstore /sys/fs/pstore
              # wait up to 1.5 seconds for the backend to be registered and the files to appear. a systemd path unit cannot detect this happening; and succeeding after a restart would not start dependent units.
              TRIES=15
              while [ "$(cat /sys/module/pstore/parameters/backend)" = "(null)" ]; do
                if (( $TRIES )); then
                  sleep 0.1
                  TRIES=$((TRIES-1))
                else
                  echo "Persistent Storage backend was not registered in time." >&2
                  break
                fi
              done
            '';
            RemainAfterExit = true;
          };
          unitConfig = {
            ConditionVirtualization = "!container";
            DefaultDependencies = false; # needed to prevent a cycle
          };
          before = [ "systemd-pstore.service" ];
          wantedBy = [ "systemd-pstore.service" ];
        };
      };

    systemd.tmpfiles.rules = [
      "d /run/keys 0750 root ${toString config.ids.gids.keys}"
      "z /run/keys 0750 root ${toString config.ids.gids.keys}"
    ];

    # Sync mount options with systemd's src/core/mount-setup.c: mount_table.
    boot.specialFileSystems = {
      "/proc" = { fsType = "proc"; options = [ "nosuid" "noexec" "nodev" ]; };
      "/run" = { fsType = "tmpfs"; options = [ "nosuid" "nodev" "strictatime" "mode=755" "size=${config.boot.runSize}" ]; };
      "/dev" = { fsType = "devtmpfs"; options = [ "nosuid" "strictatime" "mode=755" "size=${config.boot.devSize}" ]; };
      "/dev/shm" = { fsType = "tmpfs"; options = [ "nosuid" "nodev" "strictatime" "mode=1777" "size=${config.boot.devShmSize}" ]; };
      "/dev/pts" = { fsType = "devpts"; options = [ "nosuid" "noexec" "mode=620" "ptmxmode=0666" "gid=${toString config.ids.gids.tty}" ]; };

      # To hold secrets that shouldn't be written to disk
      "/run/keys" = { fsType = "ramfs"; options = [ "nosuid" "nodev" "mode=750" ]; };
    } // optionalAttrs (!config.boot.isContainer) {
      # systemd-nspawn populates /sys by itself, and remounting it causes all
      # kinds of weird issues (most noticeably, waiting for host disk device
      # nodes).
      "/sys" = { fsType = "sysfs"; options = [ "nosuid" "noexec" "nodev" ]; };
    };

  };

}