ensure we never start the same wrapper more than once

git-svn-id: https://xpra.org/svn/Xpra/trunk@10854 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/net/subprocess_wrapper.py

@@ -324,10 +324,14 @@ def subprocess_exit(self, *args):
         self._fire_callback("exit")
 
     def start(self):
+        self.start = self.fail_start
         self.process = self.exec_subprocess()
         self.protocol = self.make_protocol()
         self.protocol.start()
 
+    def fail_start(self):
+        raise Exception("this wrapper has already been started")
+
     def abort_test(self, action):
         p = self.process
         if p is None or p.poll():