This patch lets GLib's GDesktopAppInfo API watch and notice changes
to the Guix user and system profiles.  That way, the list of available
applications shown by the desktop environment is immediately updated
when the user runs "guix install", "guix remove", or "guix system
reconfigure" (see <https://issues.guix.gnu.org/35594>).

It does so by monitoring /var/guix/profiles (for changes to the system
profile) and /var/guix/profiles/per-user/USER (for changes to the user
profile) and crawling their share/applications sub-directory when
changes happen.

diff --git a/gio/gdesktopappinfo.c b/gio/gdesktopappinfo.c
index f1e2fdd..095c110 100644
--- a/gio/gdesktopappinfo.c
+++ b/gio/gdesktopappinfo.c
@@ -148,6 +148,7 @@ typedef struct
   gchar                      *alternatively_watching;
   gboolean                    is_config;
   gboolean                    is_setup;
+  gchar                      *guix_profile_watch_dir;
   GFileMonitor               *monitor;
   GHashTable                 </td></tr></table>
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modifications to their
+         * /share sub-directory.  */
+        const gchar *user;
+        DesktopFileDir *system_profile_dir, *user_profile_dir;
+
+        system_profile_dir =
+          desktop_file_dir_new ("/var/guix/profiles/system/profile/share");
+        system_profile_dir->guix_profile_watch_dir = g_strdup ("/var/guix/profiles");
+        g_ptr_array_add (desktop_file_dirs, desktop_file_dir_ref (system_profile_dir));
+
+        user = g_get_user_name ();
+        if (user != NULL)
+          {
+            gchar *profile_dir, *user_data_dir;
+
+            profile_dir = g_build_filename ("/var/guix/profiles/per-user", user, NULL);
+            user_data_dir = g_build_filename (profile_dir, "guix-profile", "share", NULL);
+            user_profile_dir = desktop_file_dir_new (user_data_dir);
+            user_profile_dir->guix_profile_watch_dir = profile_dir;
+            g_ptr_array_add (desktop_file_dirs, desktop_file_dir_ref (user_profile_dir));
+            g_free (user_data_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;