use robot description from parameter instead of topic if provided

[christianrauch]

Description

This adds the robot_description parameter back, allowing to simplify something like:

from launch import LaunchDescription
from launch_ros.actions import Node

URDF = """<?xml version="1.0"?><robot name="minimal"><link name="base_link"/><ros2_control name="FakeSystem" type="system"><hardware><plugin>mock_components/GenericSystem</plugin></hardware></ros2_control></robot>"""


def generate_launch_description():
    return LaunchDescription(
        [
            Node(
                package="robot_state_publisher",
                executable="robot_state_publisher",
                parameters=[{"robot_description": URDF}],
            ),
            Node(
                package="controller_manager",
                executable="ros2_control_node",
            ),
        ]
    )

to:

from launch import LaunchDescription
from launch_ros.actions import Node

URDF = """<?xml version="1.0"?><robot name="minimal"><link name="base_link"/><ros2_control name="FakeSystem" type="system"><hardware><plugin>mock_components/GenericSystem</plugin></hardware></ros2_control></robot>"""


def generate_launch_description():
    return LaunchDescription(
        [
            Node(
                package="controller_manager",
                executable="ros2_control_node",
                parameters=[{"robot_description": URDF}],
            )
        ]
    )

or even:

/opt/ros/lyrical/lib/controller_manager/ros2_control_node --ros-args -p robot_description:='<?xml version="1.0"?><robot name="minimal"><link name="base_link"/><ros2_control name="FakeSystem" type="system"><hardware><plugin>mock_components/GenericSystem</plugin></hardware></ros2_control></robot>'

which makes the ros2_control_node start and stop super fast.

I am not adding documentation or tests just now and only want to get feeback on the functionality.

Fixes #3480

Is this user-facing behavior change?

Yes. It adds a new parameter.

Did you use Generative AI?

No.

Additional Information

I will add documentation etc. if this gets accepted.

TODOs

To send us a pull request, please:

• Fork the repository.
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• Ensure local tests pass. (colcon test and pre-commit run (requires you to install pre-commit by pip3 install pre-commit)
• Commit to your fork using clear commit messages.
• Send a pull request, answering any default questions in the pull request interface.
• Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.

[saikishor]

After we conclude this approach. If we decide to move further, then I will be asking you to add a test case for this change.