Hub: better handle_system_error

make sure we deliver exception to MAIN greenlet even if loop.run_callback fails

gevent/hub.py

@@ -320,10 +320,15 @@ class Hub(greenlet):
         else:
             # in case system error was handled and life goes on
             # switch back to this greenlet as well
+            cb = None
+            try:
                 cb = self.loop.run_callback(current.switch)
+            except:
+                traceback.print_exc()
             try:
                 self.parent.throw(type, value)
             finally:
+                if cb is not None:
                     cb.stop()
 
     def print_exception(self, context, type, value, tb):