nixos/iso-image: Support systemd initrd (#291750)

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -4,6 +4,7 @@
 {
   config,
   lib,
+  utils,
   pkgs,
   ...
 }:
@@ -785,7 +786,11 @@ in
     # specified on the kernel command line, created in the stage 1
     # init script.
     "/iso" = lib.mkImageMediaOverride {
-      device = "/dev/root";
+      device =
+        if config.boot.initrd.systemd.enable then
+          "/dev/disk/by-label/${config.isoImage.volumeID}"
+        else
+          "/dev/root";
       neededForBoot = true;
       noCheck = true;
     };
@@ -794,7 +799,7 @@ in
     # image) to make this a live CD.
     "/nix/.ro-store" = lib.mkImageMediaOverride {
       fsType = "squashfs";
-      device = "/iso/nix-store.squashfs";
+      device = "${lib.optionalString config.boot.initrd.systemd.enable "/sysroot"}/iso/nix-store.squashfs";
       options = [
         "loop"
       ]
@@ -809,18 +814,11 @@ in
     };
 
     "/nix/store" = lib.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"
-      ];
+      overlay = {
+        lowerdir = [ "/nix/.ro-store" ];
+        upperdir = "/nix/.rw-store/store";
+        workdir = "/nix/.rw-store/work";
+      };
     };
   };
 
@@ -882,9 +880,9 @@ in
     # 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=${config.isoImage.volumeID}"
+    boot.kernelParams = lib.optionals (!config.boot.initrd.systemd.enable) [
       "boot.shell_on_fail"
+      "root=LABEL=${config.isoImage.volumeID}"
     ];
 
     fileSystems = config.lib.isoFileSystems;
@@ -901,6 +899,44 @@ in
       "overlay"
     ];
 
+    boot.initrd.systemd = lib.mkIf config.boot.initrd.systemd.enable {
+      emergencyAccess = true;
+
+      # Most of util-linux is not included by default.
+      initrdBin = [ config.boot.initrd.systemd.package.util-linux ];
+      services.copytoram = {
+        description = "Copy ISO contents to RAM";
+        requiredBy = [ "initrd.target" ];
+        before = [
+          "${utils.escapeSystemdPath "/sysroot/nix/.ro-store"}.mount"
+          "initrd-switch-root.target"
+        ];
+        unitConfig = {
+          RequiresMountsFor = "/sysroot/iso";
+          ConditionKernelCommandLine = "copytoram";
+        };
+        serviceConfig = {
+          Type = "oneshot";
+          RemainAfterExit = true;
+        };
+        path = [
+          pkgs.coreutils
+          config.boot.initrd.systemd.package.util-linux
+        ];
+        script = ''
+          device=$(findmnt -n -o SOURCE --target /sysroot/iso)
+          fsSize=$(blockdev --getsize64 "$device" || stat -Lc '%s' "$device")
+          mkdir -p /tmp-iso
+          mount --bind --make-private /sysroot/iso /tmp-iso
+          umount /sysroot/iso
+          mount -t tmpfs -o size="$fsSize" tmpfs /sysroot/iso
+          cp -r /tmp-iso/* /sysroot/iso/
+          umount /tmp-iso
+          rm -r /tmp-iso
+        '';
+      };
+    };
+
     # Closures to be copied to the Nix store on the CD, namely the init
     # script and the top-level system configuration directory.
     isoImage.storeContents = [
@@ -958,15 +994,19 @@ in
           source = "${efiDir}/EFI";
           target = "/EFI";
         }
-        {
-          source = (pkgs.writeTextDir "grub/loopback.cfg" "source /EFI/BOOT/grub.cfg") + "/grub";
-          target = "/boot/grub";
-        }
         {
           source = config.isoImage.efiSplashImage;
           target = "/EFI/BOOT/efi-background.png";
         }
       ]
+      ++ lib.optionals (config.isoImage.makeEfiBootable && !config.boot.initrd.systemd.enable) [
+        # http://www.supergrubdisk.org/wiki/Loopback.cfg
+        # This feature will be removed, and thus is not supported by systemd initrd
+        {
+          source = (pkgs.writeTextDir "grub/loopback.cfg" "source /EFI/BOOT/grub.cfg") + "/grub";
+          target = "/boot/grub";
+        }
+      ]
       ++ lib.optionals (config.boot.loader.grub.memtest86.enable && config.isoImage.makeBiosBootable) [
         {
           source = "${pkgs.memtest86plus}/memtest.bin";