bus: Avoid to hang the process with SIGUSR1 signal

Seems multiple `Xephyr -query` to a remote GDM login session causes
to leave the some processes of ibus-daemon even if the Xephyr sessions
are terminated and the issue is caused by calling bus_server_quit()
with SIGUSR1 signal.

I cannot reproduce the issue but calling the unsafe code from a signal
handler might lead to hangs and this is another signal connection
not to hang ibus-daemon.

BUG=#2499

bus/main.c

@@ -2,8 +2,8 @@
 /* vim:set et sts=4: */
 /* ibus - The Input Bus
  * Copyright (C) 2008-2013 Peng Huang <shawn.p.huang@gmail.com>
- * Copyright (C) 2013-2018 Takao Fujiwara <takao.fujiwara1@gmail.com>
- * Copyright (C) 2008-2018 Red Hat, Inc.
+ * Copyright (C) 2013-2023 Takao Fujiwara <takao.fujiwara1@gmail.com>
+ * Copyright (C) 2008-2023 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -166,11 +166,12 @@ daemon (gint nochdir, gint noclose)
 #endif
 
 #ifdef HAVE_SYS_PRCTL_H
-static void
-_sig_usr1_handler (int sig)
+static gboolean
+_on_sigusr1 (void)
 {
     g_warning ("The parent process died.");
     bus_server_quit (FALSE);
+    return G_SOURCE_REMOVE;
 }
 #endif
 
@@ -350,7 +351,7 @@ main (gint argc, gchar **argv)
         if (prctl (PR_SET_PDEATHSIG, SIGUSR1))
             g_printerr ("Cannot bind SIGUSR1 for parent death\n");
         else
-            signal (SIGUSR1, _sig_usr1_handler);
+            g_unix_signal_add (SIGUSR1, (GSourceFunc)_on_sigusr1, NULL);
 #endif
     }
     bus_server_run ();