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Spinning multiple nodes across multiple threads

asked 2021-05-06 11:14:41 -0600

Chuck Claunch gravatar image

updated 2021-05-06 11:25:54 -0600

I have an application which dynamically spins up multiple nodes and needs to spin them (as they have subscribers). Quick example, stolen from here: https://answers.ros.org/question/3583...

import threading
import rclpy
rclpy.init()
node = rclpy.create_node('simple_node')
# Spin in a separate thread
thread = threading.Thread(target=rclpy.spin, args=(node, ), daemon=True)
thread.start()
rate = node.create_rate(2)
try:
    while rclpy.ok():
        print('Help me body, you are my only hope')
        rate.sleep()
except KeyboardInterrupt:
    pass
rclpy.shutdown()
thread.join()

This example works totally fine and allows the spin to happen in a separate thread (which is what I want because my nodes do other threaded things as well). The issue I get is when i add a second node and attempt to spin it, I get ValueError: generator already executing. See this second example:

import threading
import rclpy
rclpy.init()
node = rclpy.create_node('simple_node')
node2 = rclpy.create_node('simpler_node')
# Spin in a separate thread
thread = threading.Thread(target=rclpy.spin, args=(node, ), daemon=True)
thread2 = threading.Thread(target=rclpy.spin, args=(node2, ), daemon=True)
thread.start()
thread2.start()
rate = node.create_rate(2)
try:
    while rclpy.ok():
        print('Help me body, you are my only hope')
        rate.sleep()
except KeyboardInterrupt:
    pass
rclpy.shutdown()
thread.join()
thread2.join()

This results in an Exception being thrown in the second thread:

Exception in thread Thread-2:
Help me body, you are my only hope
Traceback (most recent call last):
File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
File "/usr/lib/python3.8/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/__init__.py", line 191, in spin
    executor.spin_once()
File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 703, in spin_once
    handler, entity, node = self.wait_for_ready_callbacks(timeout_sec=timeout_sec)
File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 689, in wait_for_ready_callbacks
    return next(self._cb_iter)
ValueError: generator already executing

I realize there are other methods of composing multiple nodes but as I said, the nodes I'm building do a lot of other things in threads and are spun up dynamically by another process. Is there some method I'm missing to tell rclpy to spin all nodes? It seems the rclpy.spin() method _requires_ the node argument.

edit: Quick addition. The nodes I'm building inherit from rclpy.node.Node and I also attempted to rclpy.spin_once(self) from within those nodes in their main thread (thread that does other things) and I end up at the same ValueError: generator already executing error.

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answered 2021-05-06 14:24:40 -0600

Chuck Claunch gravatar image

Of course I waited a while to post this after not finding an answer and now I have it. I just needed to use a multi-threaded executor and spin that instead of spinning the nodes individually:

import threading
import rclpy
rclpy.init()
node = rclpy.create_node('simple_node')
node2 = rclpy.create_node('simpler_node')
executor = rclpy.executors.MultiThreadedExecutor()
executor.add_node(node)
executor.add_node(node2)
# Spin in a separate thread
executor_thread = threading.Thread(target=executor.spin, daemon=True)
executor_thread.start()
rate = node.create_rate(2)
try:
    while rclpy.ok():
        print('Help me body, you are my only hope')
        rate.sleep()
except KeyboardInterrupt:
    pass
rclpy.shutdown()
executor_thread.join()
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Asked: 2021-05-06 11:14:41 -0600

Seen: 527 times

Last updated: May 06