balloon: Don't allow multiple balloon handler registrations

Multiple balloon devices don't make sense; disallow more than one
registration attempt to register handlers.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

balloon.c

@@ -36,12 +36,20 @@ static QEMUBalloonEvent *balloon_event_fn;
 static QEMUBalloonStatus *balloon_stat_fn;
 static void *balloon_opaque;
 
-void qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
-                              QEMUBalloonStatus *stat_func, void *opaque)
+int qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
+                             QEMUBalloonStatus *stat_func, void *opaque)
 {
+    if (balloon_event_fn || balloon_stat_fn || balloon_opaque) {
+        /* We're already registered one balloon handler.  How many can
+         * a guest really have?
+         */
+        error_report("Another balloon device already registered");
+        return -1;
+    }
     balloon_event_fn = event_func;
     balloon_stat_fn = stat_func;
     balloon_opaque = opaque;
+    return 0;
 }
 
 static int qemu_balloon(ram_addr_t target)

balloon.h

@@ -20,8 +20,8 @@ typedef void (QEMUBalloonEvent)(void *opaque, ram_addr_t target);
 typedef void (QEMUBalloonStatus)(void *opaque, MonitorCompletion cb,
                                  void *cb_data);
 
-void qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
-                              QEMUBalloonStatus *stat_func, void *opaque);
+int qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
+			     QEMUBalloonStatus *stat_func, void *opaque);
 
 void monitor_print_balloon(Monitor *mon, const QObject *data);
 int do_info_balloon(Monitor *mon, MonitorCompletion cb, void *opaque);