Why is the Future 'done callback' order reversed?

asked 2020-12-09 02:46:51 -0500

SmallJoeMan
116 ●5 ●19 ●14

updated 2020-12-09 02:50:51 -0500

I'd like to attach multiple callbacks to a Future provided by a ClientGoalHandle. However, when I do this I find that the callbacks are being called in reverse order. I'm a little stumped as to why this is happening. When I look through the code for rclpy.executor or rclpy.task I can only see for callback in callbacks.

Here's a snippet from my code:

@property
def logger(self):
    return self._node.get_logger()

def foo(self, *args, **kwargs):
    self.logger.info('foo')

def bar(self, *args, **kwargs):
    self.logger.info('bar')

def baz(self, *args, **kwargs):
    self.logger.info('baz')

def qux(self, *args, **kwargs):
    self.logger.info('qux')

def next(self) -> Optional[Future]:
    self._clear_cache()
    if self.goal_handle is not None:
        self.logger.info("Cancelling current guidance.")
        future = self.goal_handle.cancel_goal_async()
        future.add_done_callback(self.foo)
        future.add_done_callback(self.bar)

        self._goal_future.add_done_callback(self.baz)
        self._goal_future.add_done_callback(self.qux)

        return self._goal_future
    else:
        return None

Here is the console readout in which you can see bar is called before foo and qux is called before baz:

[simple_state_machine-2] [INFO] [1607502712.297026172] [state]: [GoalState] Cancelling current guidance.
[velocity_controller-3] [INFO] [1607502712.301784151] [vel_ctrl]: Received cancel request.
[simple_state_machine-2] [INFO] [1607502712.302841057] [state]: [GoalState] bar
[simple_state_machine-2] [INFO] [1607502712.303296070] [state]: [GoalState] foo
[velocity_controller-3] [INFO] [1607502712.397141220] [vel_ctrl]: Guidance cancelled.
[velocity_controller-3] [INFO] [1607502712.398128136] [vel_ctrl]: Goal status: 5.
[simple_state_machine-2] [INFO] [1607502712.399765926] [state]: [GoalState] qux
[simple_state_machine-2] [INFO] [1607502712.400109978] [state]: [GoalState] baz
[simple_state_machine-2] [WARN] [1607502712.400789882] [state]: [GoalState] Navigation ended with goal status code: 5 and guidance status code: 0.

I can 'fix' this by reversing the future callback order:

with future._lock:
    future._callbacks.reverse()

But it feels like a hack to mess around with private variables within the future class. Any ideas?

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from functools import partial from rclpy.task import Future def callback(name, future): print(f"I am callback {name}") f = Future() f.add_done_callback(partial(callback, 'alice')) f.add_done_callback(partial(callback, 'bob')) f.add_done_callback(partial(callback, 'charlie')) f.set_result("foobar")

import rclpy from rclpy.executors import SingleThreadedExecutor rclpy.init() s = SingleThreadedExecutor() f = Future(executor=s) f.add_done_callback(partial(callback, 'amy')) f.add_done_callback(partial(callback, 'brice')) f.add_done_callback(partial(callback, 'cole')) f.set_result("foobar") s.spin_once() s.spin_once() s.spin_once() s.spin_once() rclpy.shutdown()

import rclpy from rclpy.executors import MultiThreadedExecutor rclpy.init() s = MultiThreadedExecutor() f = Future(executor=s) for i in range(100): f.add_done_callback(partial(callback, i)) f.set_result("foobar") for i in range(100): s.spin_once() rclpy.shutdown()

Why is the Future 'done callback' order reversed?

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