gdk: Consider XDG_ACTIVATION_TOKEN for the startup notification id

We prefer it over the old DESKTOP_STARTUP_ID environment variable if we
have it and it is valid.

We have to stash and unset XDG_ACTIVATION_TOKEN in addition to
DESKTOP_STARTUP_ID now as well. This makes sure that we don't call any
library functions which might rely on some environment variables. This
way unsetting the environment variables is safe and we can then
afterwards validate and print warnings.

gdk/gdk.c

@@ -142,31 +142,50 @@ static const GdkDebugKey gdk_debug_keys[] = {
 
 #ifdef G_HAS_CONSTRUCTORS
 #ifdef G_DEFINE_CONSTRUCTOR_NEEDS_PRAGMA
-#pragma G_DEFINE_CONSTRUCTOR_PRAGMA_ARGS(stash_desktop_startup_notification_id)
+#pragma G_DEFINE_CONSTRUCTOR_PRAGMA_ARGS(stash_and_unset_environment)
 #endif
-G_DEFINE_CONSTRUCTOR(stash_desktop_startup_notification_id)
+G_DEFINE_CONSTRUCTOR(stash_and_unset_environment)
 #endif
 
 static char *startup_notification_id = NULL;
+static char *xdg_activation_token = NULL;
 
 static void
-stash_desktop_startup_notification_id (void)
+stash_and_unset_environment (void)
 {
-  const char *desktop_startup_id;
-
-  desktop_startup_id = g_getenv ("DESKTOP_STARTUP_ID");
-  if (desktop_startup_id && *desktop_startup_id != '\0')
-    {
-      if (!g_utf8_validate (desktop_startup_id, -1, NULL))
-        g_warning ("DESKTOP_STARTUP_ID contains invalid UTF-8");
-      else
-        startup_notification_id = g_strdup (desktop_startup_id);
-    }
-
-  /* Clear the environment variable so it won't be inherited by
+  /* Copies environment variables and unsets them so they won't be inherited by
    * child processes and confuse things.
+   *
+   * Changing environment variables can be racy so we try to do this as early as
+   * possible in the program flow and before any printing that might involve
+   * environment variables.
    */
-  g_unsetenv ("DESKTOP_STARTUP_ID");
+  struct {
+    const char *key;
+    char **dst;
+  } vars[] = {
+    { "DESKTOP_STARTUP_ID", &startup_notification_id },
+    { "XDG_ACTIVATION_TOKEN", &xdg_activation_token },
+  };
+  size_t i;
+
+  for (i = 0; i < G_N_ELEMENTS (vars); i++)
+    *vars[i].dst = g_strdup (g_getenv (vars[i].key));
+
+  for (i = 0; i < G_N_ELEMENTS (vars); i++)
+    g_unsetenv (vars[i].key);
+
+  for (i = 0; i < G_N_ELEMENTS (vars); i++)
+    {
+      if (*vars[i].dst == NULL)
+        continue;
+
+      if (!g_utf8_validate (*vars[i].dst, -1, NULL))
+        {
+          g_warning ("%s contains invalid UTF-8", vars[i].key);
+          g_clear_pointer (vars[i].dst, g_free);
+        }
+    }
 }
 
 static gpointer
@@ -291,7 +310,7 @@ gdk_pre_parse (void)
     gdk_gl_backend_use (GDK_GL_WGL);
 
 #ifndef G_HAS_CONSTRUCTORS
-  stash_desktop_startup_notification_id ();
+  stash_and_unset_environment ();
 #endif
 }
 
@@ -325,15 +344,20 @@ gdk_display_open_default (void)
 /*< private >
  * gdk_get_startup_notification_id:
  *
- * Returns the original value of the DESKTOP_STARTUP_ID environment
- * variable if it was defined and valid, or %NULL otherwise.
+ * Returns the original value of the XDG_ACTIVATION_TOKEN environment
+ * variable if it was defined and valid, otherwise it returns the original
+ * value of the DESKTOP_STARTUP_ID environment variable if it was defined
+ * and valid, or %NULL if neither of them were defined and valid.
  *
  * Returns: (nullable) (transfer none): the original value of the
- *   DESKTOP_STARTUP_ID environment variable
+ *   XDG_ACTIVATION_TOKEN or DESKTOP_STARTUP_ID environment variable
  */
 const char *
 gdk_get_startup_notification_id (void)
 {
+  if (xdg_activation_token)
+    return xdg_activation_token;
+
   return startup_notification_id;
 }