working

modules/kboot-conf/default.nix

@@ -0,0 +1,60 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.boot.loader.kboot-conf;
+
+  # The builder used to write during system activation
+  builder = pkgs.substituteAll {
+    src = ./generate-kboot-conf.sh;
+    isExecutable = true;
+    path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
+    inherit (pkgs) bash;
+  };
+  # The builder exposed in populateCmd, which runs on the build architecture
+  populateBuilder = pkgs.buildPackages.substituteAll {
+    src = ./generate-kboot-conf.sh;
+    isExecutable = true;
+    path = with pkgs.buildPackages; [coreutils gnused gnugrep];
+    inherit (pkgs.buildPackages) bash;
+  };
+in
+{
+  options = {
+    boot.loader.kboot-conf = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Whether to create petitboot-compatible /kboot.conf
+        '';
+      };
+      configurationLimit = mkOption {
+        default = 10;
+        example = 5;
+        type = types.int;
+        description = ''
+          Maximum number of configurations in the generated kboot.conf.
+        '';
+      };
+      populateCmd = mkOption {
+        type = types.str;
+        readOnly = true;
+        description = ''
+          Contains the builder command used to populate an image,
+          honoring all options except the <literal>-c &lt;path-to-default-configuration&gt;</literal>
+          argument.
+          Useful to have for sdImage.populateRootCommands
+        '';
+      };
+    };
+  };
+  config = let
+    args = "-g ${toString cfg.configurationLimit} -n ${config.hardware.deviceTree.name}";
+  in mkIf cfg.enable {
+    system.build.installBootLoader = lib.mkForce "${builder} ${args} -c";
+    system.boot.loader.id = "kboot-conf";
+    boot.loader.kboot-conf.populateCmd = "${populateBuilder} ${args}";
+  };
+}

modules/kboot-conf/generate-kboot-conf.sh

@@ -0,0 +1,77 @@
+#! @bash@/bin/sh -e
+
+shopt -s nullglob
+
+export PATH=/empty
+for i in @path@; do PATH=$PATH:$i/bin; done
+
+usage() {
+    echo "usage: $0 -c <path-to-default-configuration> -n <dtbName> [-g <num-generations>] [-d <target>]" >&2
+    exit 1
+}
+
+target=/kboot.conf
+default=                # Default configuration
+numGenerations=0        # Number of other generations to include in the menu
+
+while getopts "t:c:d:g:n:" opt; do
+    case "$opt" in
+        c) default="$OPTARG" ;;
+        g) numGenerations="$OPTARG" ;;
+        d) target="$OPTARG" ;;
+        n) dtbName="$OPTARG" ;;
+        \?) usage ;;
+    esac
+done
+
+[ "$default" = "" -o "$dtbName" = "" ] && usage
+
+tmp=$target.tmp
+
+# Echo out an kboot.conf menu entry
+addEntry() {
+    local path=$(readlink -f "$1")
+    local tag="$2" # Generation number or 'default'
+
+    if ! test -e $path/kernel -a -e $path/initrd; then
+        return
+    fi
+
+    timestampEpoch=$(stat -L -c '%Z' $path)
+    timestamp=$(date "+%Y-%m-%d %H:%M" -d @$timestampEpoch)
+    nixosLabel="$(cat $path/nixos-version)"
+    extraParams="$(cat $path/kernel-params)"
+
+    local kernel=$(readlink -f "$path/kernel")
+    local initrd=$(readlink -f "$path/initrd")
+    local dtbs=$(readlink -f "$path/dtbs")
+
+    local id="nixos-$tag--$nixosLabel"
+
+    if [ "$tag" = "default" ]; then
+        echo "default=$id"
+    fi
+
+    echo -n "$id='"
+    echo -n "$kernel initrd=$initrd dtb=$dtbs/$dtbName "
+    echo -n "systemConfig=$path init=$path/init $extraParams"
+    echo "'"
+}
+
+echo "# Hola!" > $tmp
+addEntry $default default >> $tmp
+
+if [ "$numGenerations" -gt 0 ]; then
+    # Add up to $numGenerations generations of the system profile to the menu,
+    # in reverse (most recent to least recent) order.
+    for generation in $(
+            (cd /nix/var/nix/profiles && ls -d system-*-link) \
+            | sed 's/system-\([0-9]\+\)-link/\1/' \
+            | sort -n -r \
+            | head -n $numGenerations); do
+        link=/nix/var/nix/profiles/system-$generation-link
+        addEntry $link $generation
+    done >> $tmp
+fi
+
+mv -f $tmp $target