Hi Ludovic!

> +static void
> +guix_profile_changed  (GFileMonitor      *monitor,
> +                       GFile             *file,
> +                       GFile             *other_file,
> +                       GFileMonitorEvent  event_type,
> +                       gpointer           user_data)
> +{
> +  DesktopFileDir *dir = user_data;
> +
> +  desktop_file_dir_reset (dir);
> +
> +  /* When ~/.guix-profile changes, emit the 'changed' signal so everyone
> +     knows.  */
> +  g_app_info_monitor_fire ();
> +}
> +
>  static void
>  desktop_file_dir_changed (GFileMonitor      *monitor,
>                            GFile             *file,
> @@ -1531,6 +1548,7 @@ desktop_file_dirs_lock (void)
>
>    if (desktop_file_dirs == NULL || desktop_file_dirs->len == 0)
>      {
> +      const gchar *home;
>        const char * const *dirs;
>        gint i;
>
> @@ -1555,6 +1573,27 @@ desktop_file_dirs_lock (void)
>        for (i = 0; dirs[i]; i++)
>          g_ptr_array_add (desktop_file_dirs, desktop_file_dir_new (dirs[i]));
>
> +      home = g_get_home_dir ();
> +      if (guix_profile_monitor == NULL && home != NULL)
> +        {
> +          DesktopFileDir *dir;
> +          const gchar *profile, *data_dir;
> +          profile = g_build_filename (home, ".guix-profile", NULL);
> +          data_dir = g_build_filename (profile, "share", NULL);
> +          dir = desktop_file_dir_new (data_dir);
> +
> +          /* Monitor ~/.guix-profile and treat modifications to
> +             ~/.guix-profile as if they were modifications to
> +             ~/.guix-profile/share.  */
> +          guix_profile_monitor =
> +            g_local_file_monitor_new_in_worker (profile, FALSE, G_FILE_MONITOR_NONE,
> +                                                guix_profile_changed,
> +                                                desktop_file_dir_ref (dir),
> +                                                closure_notify_cb, NULL);
> +
> +          g_ptr_array_add (desktop_file_dirs, desktop_file_dir_ref (dir));
> +        }
> +
>        /* The list of directories will never change after this, unless
>         * g_get_user_config_dir() changes due to %G_TEST_OPTION_ISOLATE_DIRS. */
>        desktop_file_dirs_config_dir = user_config_dir;

I think the attached patch is an improvement over the above, by also
supporting /run/current-system/share, being a bit simpler, and not
adding extra monitors.

I've reused the cool trick of passing FALSE to
g_local_file_monitor_new_in_worker to force it to use its polling mode
fallback.