Add wip netboot image gen

net-image/make-root-fs.nix

@@ -0,0 +1,54 @@
+# Builds an ext4 image containing a populated /nix/store with the closure
+# of store paths passed in the storePaths parameter, in addition to the
+# contents of a directory that can be populated with commands. The
+# generated image is sized to only fit its contents, with the expectation
+# that a script resizes the filesystem at boot time.
+{
+  pkgs,
+  lib,
+  # List of derivations to be included
+  storePaths,
+  # Shell commands to populate the ./files directory.
+  # All files in that directory are copied to the root of the FS.
+  populateImageCommands ? "",
+  perl
+}:
+
+let
+  netbootClosureInfo = pkgs.buildPackages.closureInfo { rootPaths = storePaths; };
+in
+pkgs.stdenv.mkDerivation {
+  name = "root-fs";
+
+  nativeBuildInputs = [
+    perl
+  ];
+
+  buildCommand = ''
+    echo "Populating image with command: ${populateImageCommands}"
+    mkdir -p ./files
+    ${populateImageCommands}
+
+    echo "Preparing store paths for image..."
+    # Create nix/store before copying path
+    mkdir -p ./rootImage/nix/store
+
+    xargs -I % cp -a --reflink=auto % -t ./rootImage/nix/store/ < ${netbootClosureInfo}/store-paths
+    (
+      GLOBIGNORE=".:.."
+      shopt -u dotglob
+
+      for f in ./files/*; do
+          cp -a --reflink=auto -t ./rootImage/ "$f"
+      done
+    )
+
+    # Also include a manifest of the closures in a format suitable for nix-store --load-db
+    cp ${netbootClosureInfo}/registration ./rootImage/nix-path-registration
+
+    # done
+    echo "Image populated."
+    ls -aR ./rootImage
+    cp -r ./rootImage/ $out
+  '';
+}