Introducing ostree-grub-generator

ostree-grub-generator can be used to customize
the generated grub.cfg file. Compile time
decision ostree-grub-generator vs grub2-mkconfig
can be overwritten with the OSTREE_GRUB2_EXEC
envvar - useful for auto tests and OS installers.

Why this alternative approach:

1) The current approach is less flexible than using a
   custom 'ostree-grub-generator' script. Each system can
   adjust this script for its needs, instead of using the
   hardcoded values from ostree-bootloader-grub2.c.

2) Too much overhead on embedded to generate grub.cfg
   via /etc/grub.d/ configuration files. It is still
   possible to do so, even with this patch applied.
   No need to install grub2 package on a target device.

3) The grub2-mkconfig code path has other issues:
   https://bugzilla.gnome.org/show_bug.cgi?id=761180

Task: https://bugzilla.gnome.org/show_bug.cgi?id=762220

Closes: #228
Approved by: cgwalters

src/libostree/ostree-bootloader-grub2.c

@@ -299,8 +299,25 @@ _ostree_bootloader_grub2_write_config (OstreeBootloader      *bootloader,
   g_autofree char *bootversion_str = g_strdup_printf ("%u", (guint)bootversion);
   g_autoptr(GFile) config_path_efi_dir = NULL;
   g_autofree char *grub2_mkconfig_chroot = NULL;
+  gboolean use_system_grub2_mkconfig = TRUE;
+  const gchar *grub_exec = NULL;
 
-  if (ostree_sysroot_get_booted_deployment (self->sysroot) == NULL
+#ifdef USE_BUILTIN_GRUB2_MKCONFIG
+  use_system_grub2_mkconfig = FALSE;
+#endif
+  /* Autotests can set this envvar to select which code path to test, useful for OS installers as well */
+  grub_exec = g_getenv ("OSTREE_GRUB2_EXEC");
+  if (grub_exec)
+    {
+      if (g_str_has_suffix (grub_exec, GRUB2_MKCONFIG_PATH))
+        use_system_grub2_mkconfig = TRUE;
+      else
+        use_system_grub2_mkconfig = FALSE;
+    }
+  else
+    grub_exec = use_system_grub2_mkconfig ? GRUB2_MKCONFIG_PATH : LIBEXECDIR "/ostree-grub-generator";
+
+  if (use_system_grub2_mkconfig && ostree_sysroot_get_booted_deployment (self->sysroot) == NULL
       && g_file_has_parent (self->sysroot->path, NULL))
     {
       g_autoptr(GPtrArray) deployments = NULL;
@@ -318,6 +335,9 @@ _ostree_bootloader_grub2_write_config (OstreeBootloader      *bootloader,
        *
        * In the case of an installer, use the first deployment root (which
        * will most likely be the only one.
+       *
+       * This all only applies if we're not using the builtin
+       * generator, which handles being run outside of the root.
        */
       tool_deployment_root = ostree_sysroot_get_deployment_directory (self->sysroot, tool_deployment);
       grub2_mkconfig_chroot = g_file_get_path (tool_deployment_root);
@@ -361,7 +381,7 @@ _ostree_bootloader_grub2_write_config (OstreeBootloader      *bootloader,
      Upstream is fixed though.
   */
   proc = g_subprocess_launcher_spawn (launcher, error,
-                                      "grub2-mkconfig", "-o",
+                                      grub_exec, "-o",
                                       gs_file_get_path_cached (new_config_path),
                                       NULL);