client.call blocks the thread

[minggangw]

Bug report

Required Info:

• Operating System:
  • Ubuntu 16.04
• Installation type:
  • from source
• Version or commit hash:
  • f776f597d7d5b325ab5d49486115b5267565937e2
• DDS implementation:
• Client library (if applicable):

Steps to reproduce issue

from example_interfaces.srv import AddTwoInts

import rclpy


def main(args=None):
    rclpy.init(args=args)
    node = rclpy.create_node('minimal_client')
    cli = node.create_client(AddTwoInts, 'add_two_ints')

    req = AddTwoInts.Request()
    req.a = 41
    req.b = 1
    while not cli.wait_for_service(timeout_sec=1.0):
        node.get_logger().info('service not available, waiting again...')

    cli.call(req)

    node.get_logger().info(
        'Result of add_two_ints: for %d + %d = %d' %
        (req.a, req.b, cli.response.sum))

    node.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

Expected behavior

The client will receive the response from service side.

Actual behavior

The thread gets blocked after the line cli.call(req)

Additional information

suspicious commit 0c2ee4c

[minggangw]

After the call_async was introduced, I am not sure the code snippet above is right. Please correct me if I am wrong.

[sloretz]

Hi @minggangw thanks for the bug report. I'll close this since it looks to be working as intended. It blocks forever because there isn't an executor running. An executor must be running to unblock client.call when a service response is received.

Another detail is cli.call(req) returns the response. The client no longer has a response attribute.

The smallest change to get the code working is to replace cli.call(req) with

    future = cli.call_async(req)
    rclpy.spin_until_future_complete(node, future)
    response = future.result()

That invokes a global executor which is single-threaded. If the global executor isn't good enough for you (say you'd like callbacks to run in multiple threads), then an executor will need to be started separately.

from example_interfaces.srv import AddTwoInts
from rclpy.executors import MultiThreadedExecutor

import rclpy
import threading


def run_executor(executor, node):
    executor.add_node(node)
    try:
        executor.spin()
    finally:
        executor.shutdown()


def main(args=None):
    rclpy.init(args=args)
    node = rclpy.create_node('minimal_client')
    cli = node.create_client(AddTwoInts, 'add_two_ints')

    # Run executor that uses a thread pool in another thread so main thread is free to do what it wants
    exec = MultiThreadedExecutor(num_threads=4)
    thread = threading.Thread(target=run_executor, args=(exec, node), daemon=True)
    thread.start()

    req = AddTwoInts.Request()
    req.a = 41
    req.b = 1
    while not cli.wait_for_service(timeout_sec=1.0):
        node.get_logger().info('service not available, waiting again...')

    response = cli.call(req)

    node.get_logger().info(
        'Result of add_two_ints: for %d + %d = %d' %
        (req.a, req.b, response.sum))

    node.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

Save this as client_call_example.py

1. ros2 run examples_rclpy_minimal_service service
2. python3 client_call_example.py

See also ros2/examples an example of a custom executor and compose multiple nodes in a single process.

[minggangw]

Thanks for your explanation @sloretz .