Modularize NixOS bootloaders

nixos/bootloaders/grub-windows.nix

@@ -0,0 +1,69 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  cfg = config.myModules.bootloaders.grubWindows;
+  systemdBootCfg = config.myModules.bootloaders.systemdBoot;
+in {
+  options.myModules.bootloaders.grubWindows = {
+    enable = lib.mkEnableOption "GRUB with Windows chainloading support";
+
+    configurationLimit = lib.mkOption {
+      default = 5;
+      type = lib.types.int;
+      description = "Maximum number of NixOS configurations shown in GRUB.";
+    };
+
+    windowsEfiUuid = lib.mkOption {
+      default = null;
+      type = lib.types.nullOr lib.types.str;
+      description = ''
+        Filesystem UUID of the Windows EFI system partition. When set, GRUB
+        gets an explicit Windows Boot Manager chainload entry in addition to
+        entries discovered by os-prober.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = !systemdBootCfg.enable;
+        message = ''
+          myModules.bootloaders.grubWindows.enable conflicts with
+          myModules.bootloaders.systemdBoot.enable. Disable systemdBoot before
+          enabling the GRUB Windows boot strategy.
+        '';
+      }
+      {
+        assertion = builtins.hasAttr "/boot" config.fileSystems;
+        message = "The GRUB Windows boot strategy expects an EFI filesystem mounted at /boot.";
+      }
+    ];
+
+    boot.loader = {
+      efi = {
+        canTouchEfiVariables = lib.mkDefault true;
+        efiSysMountPoint = lib.mkDefault "/boot";
+      };
+
+      grub = {
+        enable = true;
+        efiSupport = true;
+        device = "nodev";
+        useOSProber = true;
+        configurationLimit = cfg.configurationLimit;
+        timeoutStyle = "menu";
+        extraEntries = lib.optionalString (cfg.windowsEfiUuid != null) ''
+          menuentry "Windows Boot Manager" {
+            insmod part_gpt
+            insmod fat
+            search --no-floppy --fs-uuid --set=windows_esp ${cfg.windowsEfiUuid}
+            chainloader ($windows_esp)/EFI/Microsoft/Boot/bootmgfw.efi
+          }
+        '';
+      };
+    };
+  };
+}

nixos/bootloaders/systemd-boot.nix

@@ -0,0 +1,12 @@
+{
+  config,
+  lib,
+  makeEnable,
+  ...
+}:
+makeEnable config "myModules.bootloaders.systemdBoot" true {
+  boot.loader = {
+    systemd-boot.enable = true;
+    efi.canTouchEfiVariables = lib.mkDefault true;
+  };
+}

nixos/configuration.nix

@@ -4,6 +4,8 @@
     ./android.nix
     ./base.nix
     ./ben.nix
+    ./bootloaders/grub-windows.nix
+    ./bootloaders/systemd-boot.nix
     ./cache-server.nix
     ./cache.nix
     ./code.nix

nixos/machines/ryzen-shine.nix

@@ -38,6 +38,11 @@
   # boot.kernelPackages = pkgs.linuxPackages_latest;
 
   boot.loader.systemd-boot.configurationLimit = 5;
+  myModules.bootloaders.systemdBoot.enable = false;
+  myModules.bootloaders.grubWindows = {
+    enable = true;
+    windowsEfiUuid = "B270-C7E6";
+  };
 
   networking.hostName = "ryzen-shine";
 
@@ -59,9 +64,6 @@
   networking.interfaces.enp5s0.useDHCP = true;
   networking.interfaces.wlp4s0.useDHCP = true;
 
-  boot.loader.systemd-boot.enable = true;
-  boot.loader.efi.canTouchEfiVariables = true;
-
   boot.initrd.availableKernelModules = [ "nvme" "xhci_pci" "ahci" "usb_storage" "usbhid" "sd_mod" ];
 
   hardware.nvidia.modesetting.enable = true;

nixos/raspberry-pi.nix

@@ -10,6 +10,8 @@ makeEnable config "myModules.raspberry-pi" false {
   hardware.deviceTree.filter = "bcm2711-rpi-4*.dtb";
   hardware.raspberry-pi."4".fkms-3d.enable = true;
   # hardware.raspberry-pi."4".audio.enable = true;
+  myModules.bootloaders.systemdBoot.enable = false;
+  myModules.bootloaders.grubWindows.enable = false;
 
   boot = {
     initrd.systemd.tpm2.enable = false;

nixos/wsl.nix

@@ -6,6 +6,8 @@ makeEnable config "myModules.wsl" false {
 
   myModules.base.enable = false;
   myModules.desktop.enable = false;
+  myModules.bootloaders.systemdBoot.enable = false;
+  myModules.bootloaders.grubWindows.enable = false;
   myModules.xmonad.enable = false;
   myModules.plasma.enable = false;
   # Tailscale-in-WSL is usually better handled by the Windows host Tailscale