# This module creates a bootable ISO image containing the given NixOS
# configuration.  The derivation for the ISO image will be placed in
# config.system.build.isoImage.

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

with lib;

let
  /**
   * Given a list of `options`, concats the result of mapping each options
   * to a menuentry for use in grub.
   *
   *  * defaults: {name, image, params, initrd}
   *  * options: [ option... ]
   *  * option: {name, params, class}
   */
  menuBuilderGrub2 =
  defaults: options: lib.concatStrings
    (
      map
      (option: ''
        menuentry '${defaults.name} ${
        # Name appended to menuentry defaults to params if no specific name given.
        option.name or (optionalString (option ? params) "(${option.params})")
        }' ${optionalString (option ? class) " --class ${option.class}"} {
          # Fallback to UEFI console for boot, efifb sometimes has difficulties.
          terminal_output console

          linux ${defaults.image} \''${isoboot} ${defaults.params} ${
            option.params or ""
          }
          initrd ${defaults.initrd}
        }
      '')
      options
    )
  ;

  /**
   * Builds the default options.
   */
  buildMenuGrub2 = buildMenuAdditionalParamsGrub2 "";

  targetArch =
    if config.boot.loader.grub.forcei686 then
      "ia32"
    else
      pkgs.stdenv.hostPlatform.efiArch;

  /**
   * Given params to add to `params`, build a set of default options.
   * Use this one when creating a variant (e.g. hidpi)
   */
  buildMenuAdditionalParamsGrub2 = additional:
  let
    finalCfg = {
      name = "${config.isoImage.prependToMenuLabel}${config.system.nixos.distroName} ${config.system.nixos.label}${config.isoImage.appendToMenuLabel}";
      params = "init=${config.system.build.toplevel}/init ${additional} ${toString config.boot.kernelParams}";
      image = "/boot/${config.system.boot.loader.kernelFile}";
      initrd = "/boot/initrd";
    };

  in
    menuBuilderGrub2
    finalCfg
    [
      { class = "installer"; }
      { class = "nomodeset"; params = "nomodeset"; }
      { class = "copytoram"; params = "copytoram"; }
      { class = "debug";     params = "debug"; }
    ]
  ;

  # Timeout in syslinux is in units of 1/10 of a second.
  # null means max timeout (35996, just under 1h in 1/10 seconds)
  # 0 means disable timeout
  syslinuxTimeout = if config.boot.loader.timeout == null then
      35996
    else
      config.boot.loader.timeout * 10;

  # Timeout in grub is in seconds.
  # null means max timeout (infinity)
  # 0 means disable timeout
  grubEfiTimeout = if config.boot.loader.timeout == null then
      -1
    else
      config.boot.loader.timeout;

  # The configuration file for syslinux.

  # Notes on syslinux configuration and UNetbootin compatibility:
  #   * Do not use '/syslinux/syslinux.cfg' as the path for this
  #     configuration. UNetbootin will not parse the file and use it as-is.
  #     This results in a broken configuration if the partition label does
  #     not match the specified config.isoImage.volumeID. For this reason
  #     we're using '/isolinux/isolinux.cfg'.
  #   * Use APPEND instead of adding command-line arguments directly after
  #     the LINUX entries.
  #   * COM32 entries (chainload, reboot, poweroff) are not recognized. They
  #     result in incorrect boot entries.

  baseIsolinuxCfg = ''
    SERIAL 0 115200
    TIMEOUT ${builtins.toString syslinuxTimeout}
    UI vesamenu.c32
    MENU BACKGROUND /isolinux/background.png

    ${config.isoImage.syslinuxTheme}

    DEFAULT boot

    LABEL boot
    MENU LABEL ${config.isoImage.prependToMenuLabel}${config.system.nixos.distroName} ${config.system.nixos.label}${config.isoImage.appendToMenuLabel}
    LINUX /boot/${config.system.boot.loader.kernelFile}
    APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams}
    INITRD /boot/${config.system.boot.loader.initrdFile}

    # A variant to boot with 'nomodeset'
    LABEL boot-nomodeset
    MENU LABEL ${config.isoImage.prependToMenuLabel}${config.system.nixos.distroName} ${config.system.nixos.label}${config.isoImage.appendToMenuLabel} (nomodeset)
    LINUX /boot/${config.system.boot.loader.kernelFile}
    APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} nomodeset
    INITRD /boot/${config.system.boot.loader.initrdFile}

    # A variant to boot with 'copytoram'
    LABEL boot-copytoram
    MENU LABEL ${config.isoImage.prependToMenuLabel}${config.system.nixos.distroName} ${config.system.nixos.label}${config.isoImage.appendToMenuLabel} (copytoram)
    LINUX /boot/${config.system.boot.loader.kernelFile}
    APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} copytoram
    INITRD /boot/${config.system.boot.loader.initrdFile}

    # A variant to boot with verbose logging to the console
    LABEL boot-debug
    MENU LABEL ${config.isoImage.prependToMenuLabel}${config.system.nixos.distroName} ${config.system.nixos.label}${config.isoImage.appendToMenuLabel} (debug)
    LINUX /boot/${config.system.boot.loader.kernelFile}
    APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} loglevel=7
    INITRD /boot/${config.system.boot.loader.initrdFile}

    # A variant to boot with a serial console enabled
    LABEL boot-serial
    MENU LABEL ${config.isoImage.prependToMenuLabel}${config.system.nixos.distroName} ${config.system.nixos.label}${config.isoImage.appendToMenuLabel} (serial console=ttyS0,115200n8)
    LINUX /boot/${config.system.boot.loader.kernelFile}
    APPEND init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams} console=ttyS0,115200n8
    INITRD /boot/${config.system.boot.loader.initrdFile}
  '';

  isolinuxMemtest86Entry = ''
    LABEL memtest
    MENU LABEL Memtest86+
    LINUX /boot/memtest.bin
    APPEND ${toString config.boot.loader.grub.memtest86.params}
  '';

  isolinuxCfg = concatStringsSep "\n"
    ([ baseIsolinuxCfg ] ++ optional config.boot.loader.grub.memtest86.enable isolinuxMemtest86Entry);

  refindBinary = if targetArch == "x64" || targetArch == "aa64" then "refind_${targetArch}.efi" else null;

  # Setup instructions for rEFInd.
  refind =
    if refindBinary != null then
      ''
      # Adds rEFInd to the ISO.
      cp -v ${pkgs.refind}/share/refind/${refindBinary} $out/EFI/boot/
      ''
    else
      "# No refind for ${targetArch}"
  ;

  grubPkgs = if config.boot.loader.grub.forcei686 then pkgs.pkgsi686Linux else pkgs;

  grubMenuCfg = ''
    #
    # Menu configuration
    #

    # Search using a "marker file"
    search --set=root --file /EFI/nixos-installer-image

    insmod gfxterm
    insmod png
    set gfxpayload=keep
    set gfxmode=${concatStringsSep "," [
      # GRUB will use the first valid mode listed here.
      # `auto` will sometimes choose the smallest valid mode it detects.
      # So instead we'll list a lot of possibly valid modes :/
      #"3840x2160"
      #"2560x1440"
      "1920x1200"
      "1920x1080"
      "1366x768"
      "1280x800"
      "1280x720"
      "1200x1920"
      "1024x768"
      "800x1280"
      "800x600"
      "auto"
    ]}

    if [ "\$textmode" == "false" ]; then
      terminal_output gfxterm
      terminal_input  console
    else
      terminal_output console
      terminal_input  console
      # Sets colors for console term.
      set menu_color_normal=cyan/blue
      set menu_color_highlight=white/blue
    fi

    ${ # When there is a theme configured, use it, otherwise use the background image.
    if config.isoImage.grubTheme != null then ''
      # Sets theme.
      set theme=(\$root)/EFI/boot/grub-theme/theme.txt
      # Load theme fonts
      $(find ${config.isoImage.grubTheme} -iname '*.pf2' -printf "loadfont (\$root)/EFI/boot/grub-theme/%P\n")
    '' else ''
      if background_image (\$root)/EFI/boot/efi-background.png; then
        # Black background means transparent background when there
        # is a background image set... This seems undocumented :(
        set color_normal=black/black
        set color_highlight=white/blue
      else
        # Falls back again to proper colors.
        set menu_color_normal=cyan/blue
        set menu_color_highlight=white/blue
      fi
    ''}
  '';

  # The EFI boot image.
  # Notes about grub:
  #  * Yes, the grubMenuCfg has to be repeated in all submenus. Otherwise you
  #    will get white-on-black console-like text on sub-menus. *sigh*
  efiDir = pkgs.runCommand "efi-directory" {
    nativeBuildInputs = [ pkgs.buildPackages.grub2_efi ];
    strictDeps = true;
  } ''
    mkdir -p $out/EFI/boot/

    # Add a marker so GRUB can find the filesystem.
    touch $out/EFI/nixos-installer-image

    # ALWAYS required modules.
    MODULES=(
      # Basic modules for filesystems and partition schemes
      "fat"
      "iso9660"
      "part_gpt"
      "part_msdos"

      # Basic stuff
      "normal"
      "boot"
      "linux"
      "configfile"
      "loopback"
      "chain"
      "halt"

      # Allows rebooting into firmware setup interface
      "efifwsetup"

      # EFI Graphics Output Protocol
      "efi_gop"

      # User commands
      "ls"

      # System commands
      "search"
      "search_label"
      "search_fs_uuid"
      "search_fs_file"
      "echo"

      # We're not using it anymore, but we'll leave it in so it can be used
      # by user, with the console using "C"
      "serial"

      # Graphical mode stuff
      "gfxmenu"
      "gfxterm"
      "gfxterm_background"
      "gfxterm_menu"
      "test"
      "loadenv"
      "all_video"
      "videoinfo"

      # File types for graphical mode
      "png"
    )

    echo "Building GRUB with modules:"
    for mod in ''${MODULES[@]}; do
      echo " - $mod"
    done

    # Modules that may or may not be available per-platform.
    echo "Adding additional modules:"
    for mod in efi_uga; do
      if [ -f ${grubPkgs.grub2_efi}/lib/grub/${grubPkgs.grub2_efi.grubTarget}/$mod.mod ]; then
        echo " - $mod"
        MODULES+=("$mod")
      fi
    done

    # Make our own efi program, we can't rely on "grub-install" since it seems to
    # probe for devices, even with --skip-fs-probe.
    grub-mkimage \
      --directory=${grubPkgs.grub2_efi}/lib/grub/${grubPkgs.grub2_efi.grubTarget} \
      -o $out/EFI/boot/boot${targetArch}.efi \
      -p /EFI/boot \
      -O ${grubPkgs.grub2_efi.grubTarget} \
      ''${MODULES[@]}
    cp ${grubPkgs.grub2_efi}/share/grub/unicode.pf2 $out/EFI/boot/

    cat <<EOF > $out/EFI/boot/grub.cfg

    set textmode=${boolToString (config.isoImage.forceTextMode)}
    set timeout=${toString grubEfiTimeout}

    clear
    # This message will only be viewable on the default (UEFI) console.
    echo ""
    echo "Loading graphical boot menu..."
    echo ""
    echo "Press 't' to use the text boot menu on this console..."
    echo ""

    ${grubMenuCfg}

    hiddenentry 'Text mode' --hotkey 't' {
      loadfont (\$root)/EFI/boot/unicode.pf2
      set textmode=true
      terminal_output console
    }
    hiddenentry 'GUI mode' --hotkey 'g' {
      $(find ${config.isoImage.grubTheme} -iname '*.pf2' -printf "loadfont (\$root)/EFI/boot/grub-theme/%P\n")
      set textmode=false
      terminal_output gfxterm
    }


    # If the parameter iso_path is set, append the findiso parameter to the kernel
    # line. We need this to allow the nixos iso to be booted from grub directly.
    if [ \''${iso_path} ] ; then
      set isoboot="findiso=\''${iso_path}"
    fi

    #
    # Menu entries
    #

    ${buildMenuGrub2}
    submenu "HiDPI, Quirks and Accessibility" --class hidpi --class submenu {
      ${grubMenuCfg}
      submenu "Suggests resolution @720p" --class hidpi-720p {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "video=1280x720@60"}
      }
      submenu "Suggests resolution @1080p" --class hidpi-1080p {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "video=1920x1080@60"}
      }

      # If we boot into a graphical environment where X is autoran
      # and always crashes, it makes the media unusable. Allow the user
      # to disable this.
      submenu "Disable display-manager" --class quirk-disable-displaymanager {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "systemd.mask=display-manager.service"}
      }

      # Some laptop and convertibles have the panel installed in an
      # inconvenient way, rotated away from the keyboard.
      # Those entries makes it easier to use the installer.
      submenu "" {return}
      submenu "Rotate framebuffer Clockwise" --class rotate-90cw {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "fbcon=rotate:1"}
      }
      submenu "Rotate framebuffer Upside-Down" --class rotate-180 {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "fbcon=rotate:2"}
      }
      submenu "Rotate framebuffer Counter-Clockwise" --class rotate-90ccw {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "fbcon=rotate:3"}
      }

      # As a proof of concept, mainly. (Not sure it has accessibility merits.)
      submenu "" {return}
      submenu "Use black on white" --class accessibility-blakconwhite {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "vt.default_red=0xFF,0xBC,0x4F,0xB4,0x56,0xBC,0x4F,0x00,0xA1,0xCF,0x84,0xCA,0x8D,0xB4,0x84,0x68 vt.default_grn=0xFF,0x55,0xBA,0xBA,0x4D,0x4D,0xB3,0x00,0xA0,0x8F,0xB3,0xCA,0x88,0x93,0xA4,0x68 vt.default_blu=0xFF,0x58,0x5F,0x58,0xC5,0xBD,0xC5,0x00,0xA8,0xBB,0xAB,0x97,0xBD,0xC7,0xC5,0x68"}
      }

      # Serial access is a must!
      submenu "" {return}
      submenu "Serial console=ttyS0,115200n8" --class serial {
        ${grubMenuCfg}
        ${buildMenuAdditionalParamsGrub2 "console=ttyS0,115200n8"}
      }
    }

    ${lib.optionalString (refindBinary != null) ''
    # GRUB apparently cannot do "chainloader" operations on "CD".
    if [ "\$root" != "cd0" ]; then
      menuentry 'rEFInd' --class refind {
        # Force root to be the FAT partition
        # Otherwise it breaks rEFInd's boot
        search --set=root --no-floppy --fs-uuid 1234-5678
        chainloader (\$root)/EFI/boot/${refindBinary}
      }
    fi
    ''}
    menuentry 'Firmware Setup' --class settings {
      fwsetup
      clear
      echo ""
      echo "If you see this message, your EFI system doesn't support this feature."
      echo ""
    }
    menuentry 'Shutdown' --class shutdown {
      halt
    }
    EOF

    grub-script-check $out/EFI/boot/grub.cfg

    ${refind}
  '';

  efiImg = pkgs.runCommand "efi-image_eltorito" {
    nativeBuildInputs = [ pkgs.buildPackages.mtools pkgs.buildPackages.libfaketime pkgs.buildPackages.dosfstools ];
    strictDeps = true;
  }
    # Be careful about determinism: du --apparent-size,
    #   dates (cp -p, touch, mcopy -m, faketime for label), IDs (mkfs.vfat -i)
    ''
      mkdir ./contents && cd ./contents
      mkdir -p ./EFI/boot
      cp -rp "${efiDir}"/EFI/boot/{grub.cfg,*.efi} ./EFI/boot

      # Rewrite dates for everything in the FS
      find . -exec touch --date=2000-01-01 {} +

      # Round up to the nearest multiple of 1MB, for more deterministic du output
      usage_size=$(( $(du -s --block-size=1M --apparent-size . | tr -cd '[:digit:]') * 1024 * 1024 ))
      # Make the image 110% as big as the files need to make up for FAT overhead
      image_size=$(( ($usage_size * 110) / 100 ))
      # Make the image fit blocks of 1M
      block_size=$((1024*1024))
      image_size=$(( ($image_size / $block_size + 1) * $block_size ))
      echo "Usage size: $usage_size"
      echo "Image size: $image_size"
      truncate --size=$image_size "$out"
      mkfs.vfat --invariant -i 12345678 -n EFIBOOT "$out"

      # Force a fixed order in mcopy for better determinism, and avoid file globbing
      for d in $(find EFI -type d | sort); do
        faketime "2000-01-01 00:00:00" mmd -i "$out" "::/$d"
      done

      for f in $(find EFI -type f | sort); do
        mcopy -pvm -i "$out" "$f" "::/$f"
      done

      # Verify the FAT partition.
      fsck.vfat -vn "$out"
    ''; # */

in

{
  options = {

    isoImage.isoName = mkOption {
      default = "${config.isoImage.isoBaseName}.iso";
      type = lib.types.str;
      description = lib.mdDoc ''
        Name of the generated ISO image file.
      '';
    };

    isoImage.isoBaseName = mkOption {
      default = config.system.nixos.distroId;
      type = lib.types.str;
      description = lib.mdDoc ''
        Prefix of the name of the generated ISO image file.
      '';
    };

    isoImage.compressImage = mkOption {
      default = false;
      type = lib.types.bool;
      description = lib.mdDoc ''
        Whether the ISO image should be compressed using
        {command}`zstd`.
      '';
    };

    isoImage.squashfsCompression = mkOption {
      default = with pkgs.stdenv.hostPlatform; "xz -Xdict-size 100% "
                + lib.optionalString isx86 "-Xbcj x86"
                # Untested but should also reduce size for these platforms
                + lib.optionalString isAarch "-Xbcj arm"
                + lib.optionalString (isPower && is32bit && isBigEndian) "-Xbcj powerpc"
                + lib.optionalString (isSparc) "-Xbcj sparc";
      type = lib.types.str;
      description = lib.mdDoc ''
        Compression settings to use for the squashfs nix store.
      '';
      example = "zstd -Xcompression-level 6";
    };

    isoImage.edition = mkOption {
      default = "";
      type = lib.types.str;
      description = lib.mdDoc ''
        Specifies which edition string to use in the volume ID of the generated
        ISO image.
      '';
    };

    isoImage.volumeID = mkOption {
      # nixos-$EDITION-$RELEASE-$ARCH
      default = "nixos${optionalString (config.isoImage.edition != "") "-${config.isoImage.edition}"}-${config.system.nixos.release}-${pkgs.stdenv.hostPlatform.uname.processor}";
      type = lib.types.str;
      description = lib.mdDoc ''
        Specifies the label or volume ID of the generated ISO image.
        Note that the label is used by stage 1 of the boot process to
        mount the CD, so it should be reasonably distinctive.
      '';
    };

    isoImage.contents = mkOption {
      example = literalExpression ''
        [ { source = pkgs.memtest86 + "/memtest.bin";
            target = "boot/memtest.bin";
          }
        ]
      '';
      description = lib.mdDoc ''
        This option lists files to be copied to fixed locations in the
        generated ISO image.
      '';
    };

    isoImage.storeContents = mkOption {
      example = literalExpression "[ pkgs.stdenv ]";
      description = lib.mdDoc ''
        This option lists additional derivations to be included in the
        Nix store in the generated ISO image.
      '';
    };

    isoImage.includeSystemBuildDependencies = mkOption {
      default = false;
      type = lib.types.bool;
      description = lib.mdDoc ''
        Set this option to include all the needed sources etc in the
        image. It significantly increases image size. Use that when
        you want to be able to keep all the sources needed to build your
        system or when you are going to install the system on a computer
        with slow or non-existent network connection.
      '';
    };

    isoImage.makeBiosBootable = mkOption {
      # Before this option was introduced, images were BIOS-bootable if the
      # hostPlatform was x86-based. This option is enabled by default for
      # backwards compatibility.
      #
      # Also note that syslinux package currently cannot be cross-compiled from
      # non-x86 platforms, so the default is false on non-x86 build platforms.
      default = pkgs.stdenv.buildPlatform.isx86 && pkgs.stdenv.hostPlatform.isx86;
      defaultText = lib.literalMD ''
        `true` if both build and host platforms are x86-based architectures,
        e.g. i686 and x86_64.
      '';
      type = lib.types.bool;
      description = lib.mdDoc ''
        Whether the ISO image should be a BIOS-bootable disk.
      '';
    };

    isoImage.makeEfiBootable = mkOption {
      default = false;
      type = lib.types.bool;
      description = lib.mdDoc ''
        Whether the ISO image should be an EFI-bootable volume.
      '';
    };

    isoImage.makeUsbBootable = mkOption {
      default = false;
      type = lib.types.bool;
      description = lib.mdDoc ''
        Whether the ISO image should be bootable from CD as well as USB.
      '';
    };

    isoImage.efiSplashImage = mkOption {
      default = pkgs.fetchurl {
          url = "https://raw.githubusercontent.com/NixOS/nixos-artwork/a9e05d7deb38a8e005a2b52575a3f59a63a4dba0/bootloader/efi-background.png";
          sha256 = "18lfwmp8yq923322nlb9gxrh5qikj1wsk6g5qvdh31c4h5b1538x";
        };
      description = lib.mdDoc ''
        The splash image to use in the EFI bootloader.
      '';
    };

    isoImage.splashImage = mkOption {
      default = pkgs.fetchurl {
          url = "https://raw.githubusercontent.com/NixOS/nixos-artwork/a9e05d7deb38a8e005a2b52575a3f59a63a4dba0/bootloader/isolinux/bios-boot.png";
          sha256 = "1wp822zrhbg4fgfbwkr7cbkr4labx477209agzc0hr6k62fr6rxd";
        };
      description = lib.mdDoc ''
        The splash image to use in the legacy-boot bootloader.
      '';
    };

    isoImage.grubTheme = mkOption {
      default = pkgs.nixos-grub2-theme;
      type = types.nullOr (types.either types.path types.package);
      description = lib.mdDoc ''
        The grub2 theme used for UEFI boot.
      '';
    };

    isoImage.syslinuxTheme = mkOption {
      default = ''
        MENU TITLE ${config.system.nixos.distroName}
        MENU RESOLUTION 800 600
        MENU CLEAR
        MENU ROWS 6
        MENU CMDLINEROW -4
        MENU TIMEOUTROW -3
        MENU TABMSGROW  -2
        MENU HELPMSGROW -1
        MENU HELPMSGENDROW -1
        MENU MARGIN 0

        #                                FG:AARRGGBB  BG:AARRGGBB   shadow
        MENU COLOR BORDER       30;44      #00000000    #00000000   none
        MENU COLOR SCREEN       37;40      #FF000000    #00E2E8FF   none
        MENU COLOR TABMSG       31;40      #80000000    #00000000   none
        MENU COLOR TIMEOUT      1;37;40    #FF000000    #00000000   none
        MENU COLOR TIMEOUT_MSG  37;40      #FF000000    #00000000   none
        MENU COLOR CMDMARK      1;36;40    #FF000000    #00000000   none
        MENU COLOR CMDLINE      37;40      #FF000000    #00000000   none
        MENU COLOR TITLE        1;36;44    #00000000    #00000000   none
        MENU COLOR UNSEL        37;44      #FF000000    #00000000   none
        MENU COLOR SEL          7;37;40    #FFFFFFFF    #FF5277C3   std
      '';
      type = types.str;
      description = lib.mdDoc ''
        The syslinux theme used for BIOS boot.
      '';
    };

    isoImage.prependToMenuLabel = mkOption {
      default = "";
      type = types.str;
      example = "Install ";
      description = lib.mdDoc ''
        The string to prepend before the menu label for the NixOS system.
        This will be directly prepended (without whitespace) to the NixOS version
        string, like for example if it is set to `XXX`:

        `XXXNixOS 99.99-pre666`
      '';
    };

    isoImage.appendToMenuLabel = mkOption {
      default = " Installer";
      type = types.str;
      example = " Live System";
      description = lib.mdDoc ''
        The string to append after the menu label for the NixOS system.
        This will be directly appended (without whitespace) to the NixOS version
        string, like for example if it is set to `XXX`:

        `NixOS 99.99-pre666XXX`
      '';
    };

    isoImage.forceTextMode = mkOption {
      default = false;
      type = types.bool;
      example = true;
      description = lib.mdDoc ''
        Whether to use text mode instead of graphical grub.
        A value of `true` means graphical mode is not tried to be used.

        This is useful for validating that graphics mode usage is not at the root cause of a problem with the iso image.

        If text mode is required off-handedly (e.g. for serial use) you can use the `T` key, after being prompted, to use text mode for the current boot.
      '';
    };

  };

  # store them in lib so we can mkImageMediaOverride the
  # entire file system layout in installation media (only)
  config.lib.isoFileSystems = {
    "/" = mkImageMediaOverride
      {
        fsType = "tmpfs";
        options = [ "mode=0755" ];
      };

    # Note that /dev/root is a symlink to the actual root device
    # specified on the kernel command line, created in the stage 1
    # init script.
    "/iso" = mkImageMediaOverride
      { device = "/dev/root";
        neededForBoot = true;
        noCheck = true;
      };

    # In stage 1, mount a tmpfs on top of /nix/store (the squashfs
    # image) to make this a live CD.
    "/nix/.ro-store" = mkImageMediaOverride
      { fsType = "squashfs";
        device = "/iso/nix-store.squashfs";
        options = [ "loop" ];
        neededForBoot = true;
      };

    "/nix/.rw-store" = mkImageMediaOverride
      { fsType = "tmpfs";
        options = [ "mode=0755" ];
        neededForBoot = true;
      };

    "/nix/store" = mkImageMediaOverride
      { fsType = "overlay";
        device = "overlay";
        options = [
          "lowerdir=/nix/.ro-store"
          "upperdir=/nix/.rw-store/store"
          "workdir=/nix/.rw-store/work"
        ];
        depends = [
          "/nix/.ro-store"
          "/nix/.rw-store/store"
          "/nix/.rw-store/work"
        ];
      };
  };

  config = {
    assertions = [
      {
        # Syslinux (and isolinux) only supports x86-based architectures.
        assertion = config.isoImage.makeBiosBootable -> pkgs.stdenv.hostPlatform.isx86;
        message = "BIOS boot is only supported on x86-based architectures.";
      }
      {
        assertion = !(stringLength config.isoImage.volumeID > 32);
        # https://wiki.osdev.org/ISO_9660#The_Primary_Volume_Descriptor
        # Volume Identifier can only be 32 bytes
        message = let
          length = stringLength config.isoImage.volumeID;
          howmany = toString length;
          toomany = toString (length - 32);
        in
        "isoImage.volumeID ${config.isoImage.volumeID} is ${howmany} characters. That is ${toomany} characters longer than the limit of 32.";
      }
    ];

    # Don't build the GRUB menu builder script, since we don't need it
    # here and it causes a cyclic dependency.
    boot.loader.grub.enable = false;

    environment.systemPackages =  [ grubPkgs.grub2 grubPkgs.grub2_efi ]
      ++ optional (config.isoImage.makeBiosBootable) pkgs.syslinux
    ;

    # In stage 1 of the boot, mount the CD as the root FS by label so
    # that we don't need to know its device.  We pass the label of the
    # root filesystem on the kernel command line, rather than in
    # `fileSystems' below.  This allows CD-to-USB converters such as
    # UNetbootin to rewrite the kernel command line to pass the label or
    # UUID of the USB stick.  It would be nicer to write
    # `root=/dev/disk/by-label/...' here, but UNetbootin doesn't
    # recognise that.
    boot.kernelParams =
      [ "root=LABEL=${config.isoImage.volumeID}"
        "boot.shell_on_fail"
      ];

    fileSystems = config.lib.isoFileSystems;

    boot.initrd.availableKernelModules = [ "squashfs" "iso9660" "uas" "overlay" ];

    boot.initrd.kernelModules = [ "loop" "overlay" ];

    # Closures to be copied to the Nix store on the CD, namely the init
    # script and the top-level system configuration directory.
    isoImage.storeContents =
      [ config.system.build.toplevel ] ++
      optional config.isoImage.includeSystemBuildDependencies
        config.system.build.toplevel.drvPath;

    # Create the squashfs image that contains the Nix store.
    system.build.squashfsStore = pkgs.callPackage ../../../lib/make-squashfs.nix {
      storeContents = config.isoImage.storeContents;
      comp = config.isoImage.squashfsCompression;
    };

    # Individual files to be included on the CD, outside of the Nix
    # store on the CD.
    isoImage.contents =
      [
        { source = config.boot.kernelPackages.kernel + "/" + config.system.boot.loader.kernelFile;
          target = "/boot/" + config.system.boot.loader.kernelFile;
        }
        { source = config.system.build.initialRamdisk + "/" + config.system.boot.loader.initrdFile;
          target = "/boot/" + config.system.boot.loader.initrdFile;
        }
        { source = config.system.build.squashfsStore;
          target = "/nix-store.squashfs";
        }
        { source = pkgs.writeText "version" config.system.nixos.label;
          target = "/version.txt";
        }
      ] ++ optionals (config.isoImage.makeBiosBootable) [
        { source = config.isoImage.splashImage;
          target = "/isolinux/background.png";
        }
        { source = pkgs.substituteAll  {
            name = "isolinux.cfg";
            src = pkgs.writeText "isolinux.cfg-in" isolinuxCfg;
            bootRoot = "/boot";
          };
          target = "/isolinux/isolinux.cfg";
        }
        { source = "${pkgs.syslinux}/share/syslinux";
          target = "/isolinux";
        }
      ] ++ optionals config.isoImage.makeEfiBootable [
        { source = efiImg;
          target = "/boot/efi.img";
        }
        { source = "${efiDir}/EFI";
          target = "/EFI";
        }
        { source = (pkgs.writeTextDir "grub/loopback.cfg" "source /EFI/boot/grub.cfg") + "/grub";
          target = "/boot/grub";
        }
        { source = config.isoImage.efiSplashImage;
          target = "/EFI/boot/efi-background.png";
        }
      ] ++ optionals (config.boot.loader.grub.memtest86.enable && config.isoImage.makeBiosBootable) [
        { source = "${pkgs.memtest86plus}/memtest.bin";
          target = "/boot/memtest.bin";
        }
      ] ++ optionals (config.isoImage.grubTheme != null) [
        { source = config.isoImage.grubTheme;
          target = "/EFI/boot/grub-theme";
        }
      ];

    boot.loader.timeout = 10;

    # Create the ISO image.
    system.build.isoImage = pkgs.callPackage ../../../lib/make-iso9660-image.nix ({
      inherit (config.isoImage) isoName compressImage volumeID contents;
      bootable = config.isoImage.makeBiosBootable;
      bootImage = "/isolinux/isolinux.bin";
      syslinux = if config.isoImage.makeBiosBootable then pkgs.syslinux else null;
    } // optionalAttrs (config.isoImage.makeUsbBootable && config.isoImage.makeBiosBootable) {
      usbBootable = true;
      isohybridMbrImage = "${pkgs.syslinux}/share/syslinux/isohdpfx.bin";
    } // optionalAttrs config.isoImage.makeEfiBootable {
      efiBootable = true;
      efiBootImage = "boot/efi.img";
    });

    boot.postBootCommands =
      ''
        # After booting, register the contents of the Nix store on the
        # CD in the Nix database in the tmpfs.
        ${config.nix.package.out}/bin/nix-store --load-db < /nix/store/nix-path-registration

        # nixos-rebuild also requires a "system" profile and an
        # /etc/NIXOS tag.
        touch /etc/NIXOS
        ${config.nix.package.out}/bin/nix-env -p /nix/var/nix/profiles/system --set /run/current-system
      '';

    # Add vfat support to the initrd to enable people to copy the
    # contents of the CD to a bootable USB stick.
    boot.initrd.supportedFilesystems = [ "vfat" ];

  };

}