Fix handling of SIGCHLD

The commit that removed metacity-dialog added a global SIGCHLD handler
that caused problems by (a) calling waitpid(-1) and thus breaking
g_child_watch for everyone else, and (b) doing too much from a signal
handler and sometimes causing deadlocks (bug 596200).

This removes the global handler and has each zenity user create its
own child watch to watch for exit. (It also fixes the window class of
the zenity dialogs, so that meta_window_present_delete_dialog() will
work again.)

src/core/delete.c

@@ -55,27 +55,16 @@ delete_ping_reply_func (MetaDisplay *display,
   /* we do nothing */
 }
 
-static gboolean
-delete_window_callback (gpointer w_p)
-{
-  meta_window_kill ((MetaWindow*) w_p);
-
-  return FALSE; /* don't do it again */
-}
-
 static void
-sigchld_handler (MetaNexus *nexus, guint arg1, gpointer arg2, gpointer user_data)
+dialog_exited (GPid pid, int status, gpointer user_data)
 {
   MetaWindow *ours = (MetaWindow*) user_data;
 
-  if (GPOINTER_TO_INT (arg2) == ours->dialog_pid)
-    {
-      if (arg1 == 1 /* pressed "force quit" */)
-        g_idle_add_full (G_PRIORITY_DEFAULT,
-                         delete_window_callback, user_data, NULL);
+  ours->dialog_pid = -1;
 
-      ours->dialog_pid = -1; /* forget it anyway */
-    }
+  /* exit status of 1 means the user pressed "Force Quit" */
+  if (WIFEXITED (status) && WEXITSTATUS (status) == 1)
+    meta_window_kill (ours);
 }
 
 static void
@@ -119,11 +108,7 @@ delete_ping_timeout_func (MetaDisplay *display,
   g_free (window_content);
 
   window->dialog_pid = dialog_pid;
-
-  g_signal_connect (sigchld_nexus, "sigchld",
-                    G_CALLBACK (sigchld_handler),
-                    window);
-
+  g_child_watch_add (dialog_pid, dialog_exited, window);
 }
 
 void