Set the query callback function and user data that will be called after collectpads has received a query originating from one of the collected pads.
If the query being processed is a serialized one, this callback is called with this STREAM_LOCK held, otherwise not. As this lock should be held when calling a number of CollectPads functions, it should be acquired if so (unusually) needed.
MT safe.
| this |
the collectpads to use |
| func |
the function to set |
| user_data |
user data passed to the function |