nixos/netboot: use config.system.boot.loader.initrdFile instead of hardcoding initrd, disable compression for initrd

This verifies that we can direct boot a netboot image as expected.

nixos/modules/installer/netboot/netboot.nix

@@ -1,7 +1,7 @@
 # This module creates netboot media containing the given NixOS
 # configuration.
 
-{ config, lib, pkgs, ... }:
+{ options, config, lib, pkgs, ... }:
 
 with lib;
 
@@ -41,6 +41,34 @@ with lib;
     environment.systemPackages = [ pkgs.grub2_efi ]
       ++ (lib.optionals (pkgs.stdenv.hostPlatform.system != "aarch64-linux") [pkgs.grub2 pkgs.syslinux]);
 
+    # We only want to set those options in the context of
+    # the QEMU infrastructure.
+    virtualisation = lib.optionalAttrs (options ? virtualisation.directBoot) {
+      # By default, using netboot images in virtualized contexts
+      # should not create any disk image ideally, except if
+      # asked explicitly.
+      diskImage = mkDefault null;
+      # We do not want to mount the host Nix store in those situations.
+      mountHostNixStore = mkDefault false;
+      # We do not need the nix store image because:
+      # - either we boot through network and we have the squashfs image
+      # - either we direct boot, we have the squashfs image
+      useNixStoreImage = mkDefault false;
+      # Though, we still want a writable store through .rw-store
+      writableStore = mkDefault true;
+      # Ideally, we might not want to test the network / firmware.
+      directBoot = {
+        enable = mkDefault true;
+        # We need to use our netboot initrd which contains a copy of the Nix store.
+        initrd = "${config.system.build.netbootRamdisk}/${config.system.boot.loader.initrdFile}";
+      };
+      # We do not want to use the default filesystems.
+      useDefaultFilesystems = mkDefault false;
+      # Bump the default memory size as we are loading the whole initrd in RAM.
+      memorySize = mkDefault 1536;
+    };
+
+
     fileSystems."/" = mkImageMediaOverride
       { fsType = "tmpfs";
         options = [ "mode=0755" ];
@@ -95,8 +123,7 @@ with lib;
 
     # Create the initrd
     system.build.netbootRamdisk = pkgs.makeInitrdNG {
-      inherit (config.boot.initrd) compressor;
-      prepend = [ "${config.system.build.initialRamdisk}/initrd" ];
+      prepend = [ "${config.system.build.initialRamdisk}/${config.system.boot.loader.initrdFile}" ];
 
       contents =
         [ { object = config.system.build.squashfsStore;
@@ -109,8 +136,8 @@ with lib;
       #!ipxe
       # Use the cmdline variable to allow the user to specify custom kernel params
       # when chainloading this script from other iPXE scripts like netboot.xyz
-      kernel ${pkgs.stdenv.hostPlatform.linux-kernel.target} init=${config.system.build.toplevel}/init initrd=initrd ${toString config.boot.kernelParams} ''${cmdline}
-      initrd initrd
+      kernel ${pkgs.stdenv.hostPlatform.linux-kernel.target} init=${config.system.build.toplevel}/init initrd=${config.system.boot.loader.initrdFile} ${toString config.boot.kernelParams} ''${cmdline}
+      initrd ${config.system.boot.loader.initrdFile}
       boot
     '';
 
@@ -124,7 +151,7 @@ with lib;
       fi
       SCRIPT_DIR=$( cd -- "$( dirname -- "''${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
       kexec --load ''${SCRIPT_DIR}/bzImage \
-        --initrd=''${SCRIPT_DIR}/initrd.gz \
+        --initrd=''${SCRIPT_DIR}/${config.system.boot.loader.initrdFile} \
         --command-line "init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams}"
       kexec -e
     '';
@@ -132,8 +159,8 @@ with lib;
     # A tree containing initrd.gz, bzImage and a kexec-boot script.
     system.build.kexecTree = pkgs.linkFarm "kexec-tree" [
       {
-        name = "initrd.gz";
-        path = "${config.system.build.netbootRamdisk}/initrd";
+        name = "${config.system.boot.loader.initrdFile}";
+        path = "${config.system.build.netbootRamdisk}/${config.system.boot.loader.initrdFile}";
       }
       {
         name = "bzImage";

nixos/modules/virtualisation/qemu-vm.nix

@@ -1192,7 +1192,7 @@ in
     # value for the `fileSystems' attribute should be disregarded (since those
     # filesystems don't necessarily exist in the VM). You can disable this
     # override by setting `virtualisation.fileSystems = lib.mkForce { };`.
-    fileSystems = lib.mkIf (cfg.fileSystems != { }) (mkVMOverride cfg.fileSystems);
+    fileSystems = lib.mkIf (cfg.fileSystems != { }) (lib.mapAttrs (n: v: mkVMOverride v) cfg.fileSystems);
 
     virtualisation.fileSystems = let
       mkSharedDir = tag: share:

nixos/tests/boot.nix

@@ -104,6 +104,15 @@ in {
       bios = uefiBinary;
     };
 
+    directNetboot = makeTest {
+      name = "directboot-netboot";
+      nodes.machine = {
+        imports = [ ../modules/installer/netboot/netboot-minimal.nix ];
+        virtualisation.memorySize = 4096;
+      };
+      testScript = "machine.fail('stat /dev/vda')";
+    };
+
     uefiNetboot = makeNetbootTest "uefi" {
       bios = uefiBinary;
       # Custom ROM is needed for EFI PXE boot. I failed to understand exactly why, because QEMU should still use iPXE for EFI.