From 54129e72b4610b80843d86537c15e1ebc06628c9 Mon Sep 17 00:00:00 2001
From: Florian Klink <flokli@flokli.de>
Date: Sun, 21 Jun 2020 11:18:37 +0200
Subject: [PATCH] nixos/generic-extlinux-compatible: introduce
 boot.loader.generic-extlinux-compatible.populateCmd

This option exposes the builder command used to populate an image,
honoring all options except the -c <path-to-default-configuration>
argument.

Useful to have for sdImage.populateRootCommands.

Special care needs to be taken w.r.t cross - the populate command runs
on the host platform, the activation script on the build platform (so
the builders differ)
---
 .../generic-extlinux-compatible/default.nix   | 28 ++++++++++++++++---
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/nixos/modules/system/boot/loader/generic-extlinux-compatible/default.nix b/nixos/modules/system/boot/loader/generic-extlinux-compatible/default.nix
index af39c7bb684..5ddf96f5790 100644
--- a/nixos/modules/system/boot/loader/generic-extlinux-compatible/default.nix
+++ b/nixos/modules/system/boot/loader/generic-extlinux-compatible/default.nix
@@ -8,7 +8,10 @@ let
 
   timeoutStr = if blCfg.timeout == null then "-1" else toString blCfg.timeout;
 
+  # The builder used to write during system activation
   builder = import ./extlinux-conf-builder.nix { inherit pkgs; };
+  # The builder exposed in populateCmd, which runs on the build architecture
+  populateBuilder = import ./extlinux-conf-builder.nix { pkgs = pkgs.buildPackages; };
 in
 {
   options = {
@@ -34,11 +37,28 @@ in
           Maximum number of configurations in the boot menu.
         '';
       };
+
+      populateCmd = mkOption {
+        type = types.str;
+        readOnly = true;
+        description = ''
+          Contains the builder command used to populate an image,
+          honoring all options except the -c <path-to-default-configuration>
+          argument.
+          Useful to have for sdImage.populateRootCommands
+        '';
+      };
+
     };
   };
 
-  config = mkIf cfg.enable {
-    system.build.installBootLoader = "${builder} -g ${toString cfg.configurationLimit} -t ${timeoutStr} -c";
-    system.boot.loader.id = "generic-extlinux-compatible";
-  };
+  config = let
+    builderArgs = "-g ${toString cfg.configurationLimit} -t ${timeoutStr}";
+  in
+    mkIf cfg.enable {
+      system.build.installBootLoader = "${builder} ${builderArgs} -c";
+      system.boot.loader.id = "generic-extlinux-compatible";
+
+      boot.loader.generic-extlinux-compatible.populateCmd = "${populateBuilder} ${builderArgs}";
+    };
 }