make-disk-image: use nix path-info to automatically set the imageSize

Author: MatthewCroughan

lib/make-disk-image.nix

@@ -24,7 +24,14 @@ let
     util-linux
   ];
   preVM = ''
-    ${lib.concatMapStringsSep "\n" (disk: "truncate -s ${disk.imageSize} ${disk.name}.raw") (lib.attrValues nixosConfig.config.disko.devices.disk)}
+    export PATH=$PATH:${lib.makeBinPath (with pkgs; [ nix jq ])}
+    export NIX_STATE_DIR=$TMPDIR/state
+    export NIX_CONFIG="experimental-features = nix-command"
+    nix-store --load-db < ${pkgs.closureInfo {
+      rootPaths = [ systemToInstall.config.system.build.toplevel ];
+    }}/registration
+
+    ${lib.concatMapStringsSep "\n" (disk: "truncate -s $(($(set -x; nix path-info --json ${systemToInstall.config.system.build.toplevel} | jq .[].closureSize) + ${toString disk.imageSizeExtraBytes})) ${disk.name}.raw") (lib.attrValues nixosConfig.config.disko.devices.disk)}
   '';
   postVM = ''
     # shellcheck disable=SC2154

lib/types/disk.nix

@@ -16,13 +16,13 @@
       type = diskoLib.optionTypes.absolute-pathname; # TODO check if subpath of /dev ? - No! eg: /.swapfile
       description = "Device path";
     };
-    imageSize = lib.mkOption {
-      type = lib.types.strMatching "[0-9]+[KMGTP]?";
+    imageSizeExtraBytes = lib.mkOption {
+      type = lib.types.int;
       description = ''
-        size of the image if the makeDiskImages function from diksoLib is used.
-        is used as an argument to truncate -s
+        extra size in bytes to add to the end of the disk image if the makeDiskImages function from diksoLib is used.
+        This is used as an argument to truncate -s
       '';
-      default = "2G";
+      default = 2000000000;
     };
     content = diskoLib.deviceType { parent = config; device = config.device; };
     _meta = lib.mkOption {