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

It does so by monitoring /nix/var/nix/profiles (for changes to the system
profile) and /nix/var/nix/profiles/per-user/USER (for changes to the user
profile) as well as /etc/profiles/per-user (for chanes to the user
environment 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                      *nix_profile_watch_dir;
   GFileMonitor               *monitor;
   GHashTable                 *app_names;
   GHashTable                 *mime_tweaks;
@@ -180,6 +181,7 @@ desktop_file_dir_unref (DesktopFileDir *dir)
     {
       desktop_file_dir_reset (dir);
       g_free (dir->path);
+      g_free (dir->nix_profile_watch_dir);
       g_free (dir);
     }
 }
@@ -204,6 +206,14 @@ desktop_file_dir_get_alternative_dir (DesktopFileDir *dir)
 {
   gchar *parent;
 
+  /* If DIR is a profile, watch the specified directory--e.g.,
+   * /nix/var/nix/profiles/per-user/$USER/ for the user profile.  Do not watch
+   * ~/.nix-profile or /run/current-system/sw because GFileMonitor does
+   * not pass IN_DONT_FOLLOW and thus cannot notice any change.
+   * /etc/profiles/per-user is monitored directly for the same reason. */
+  if (dir->nix_profile_watch_dir != NULL)
+    return g_strdup (dir->nix_profile_watch_dir);
+
   /* If the directory itself exists then we need no alternative. */
   if (g_access (dir->path, R_OK | X_OK) == 0)
     return NULL;
@@ -249,11 +258,11 @@ desktop_file_dir_changed (GFileMonitor      *monitor,
    *
    * If this is a notification for a parent directory (because the
    * desktop directory didn't exist) then we shouldn't fire the signal
-   * unless something actually changed.
+   * unless something actually changed or it's part of a Nix profile.
    */
   g_mutex_lock (&desktop_file_dir_lock);
 
-  if (dir->alternatively_watching)
+  if (dir->alternatively_watching && dir->nix_profile_watch_dir == NULL)
     {
       gchar *alternative_dir;
 
@@ -1555,6 +1564,40 @@ desktop_file_dirs_lock (void)
       for (i = 0; dirs[i]; i++)
         g_ptr_array_add (desktop_file_dirs, desktop_file_dir_new (dirs[i]));
 
+      {
+        /* Monitor the system and user profile under /nix/var/nix/profiles and
+         * treat modifications to them as if they were modifications to their
+         * /share sub-directory.  */
+        const gchar *user;
+        DesktopFileDir *system_profile_dir, *user_profile_dir, *user_env_dir;
+
+        system_profile_dir =
+          desktop_file_dir_new ("/nix/var/nix/profiles/system/sw/share");
+        system_profile_dir->nix_profile_watch_dir = g_strdup ("/nix/var/nix/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, *env_dir, *env_data_dir;
+
+            profile_dir = g_build_filename ("/nix/var/nix/profiles/per-user", user, NULL);
+            user_data_dir = g_build_filename (profile_dir, "profile", "share", NULL);
+            user_profile_dir = desktop_file_dir_new (user_data_dir);
+            user_profile_dir->nix_profile_watch_dir = profile_dir;
+            
+            env_dir = g_build_filename ("/etc/profiles/per-user", NULL);
+            env_data_dir = g_build_filename (env_dir, user, "share", NULL);
+            user_env_dir = desktop_file_dir_new (env_data_dir);
+            user_env_dir->nix_profile_watch_dir = env_dir;
+
+            g_ptr_array_add (desktop_file_dirs, desktop_file_dir_ref (user_profile_dir));
+            g_ptr_array_add (desktop_file_dirs, desktop_file_dir_ref (user_env_dir));
+            g_free (user_data_dir);
+            g_free (env_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;