Namespaced tags for gazebo sdf are raising parsing issues

[tomkimsour]

Environment

• OS Version: Ubuntu 20.04, ROS2 Humble

• Binary build?
  Gazebo 6.16

  • Generally, mention all circumstances that might affect rendering capabilities:
    • running on a dual GPU machine (integrated GPU + discrete GPU)
    • running in Docker/Singularity

robot_description = load_xacro(xacro_file_path, xacro_input_args)
  File "/opt/ros/humble/lib/python3.10/site-packages/launch_param_builder/utils.py", line 82, in load_xacro
    file = xacro.process_file(file_path, mappings=mappings)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 1098, in process_file
    process_doc(doc, **kwargs)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 1050, in process_doc
    eval_all(doc.documentElement, macros, symbols)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 931, in eval_all
    process_include(node, macros, symbols, eval_all)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 522, in process_include
    func(include, ns_macros, ns_symbols)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 931, in eval_all
    process_include(node, macros, symbols, eval_all)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 522, in process_include
    func(include, ns_macros, ns_symbols)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 931, in eval_all
    process_include(node, macros, symbols, eval_all)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 522, in process_include
    func(include, ns_macros, ns_symbols)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 931, in eval_all
    process_include(node, macros, symbols, eval_all)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 519, in process_include
    include = parse(None, filename).documentElement
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/xacro/__init__.py", line 1022, in parse
    return xml.dom.minidom.parse(inp)
  File "/usr/lib/python3.10/xml/dom/minidom.py", line 1990, in parse
    return expatbuilder.parse(file)
  File "/usr/lib/python3.10/xml/dom/expatbuilder.py", line 913, in parse
    result = builder.parseFile(file)
  File "/usr/lib/python3.10/xml/dom/expatbuilder.py", line 207, in parseFile
    parser.Parse(buffer, False)
xml.parsers.expat.ExpatError: unbound prefix: line 48, column 16

Description

• Expected behavior: Generate URDF correctly from xacro despite having special tags from gazebo
• Actual behavior: robot_state_publisher fails to parse the urdf due to tag ignition:expressed_in

Steps to reproduce

1. In a xacro file of the wheels of a robot, define the collision sdf as followed :

     <collision>
        <surface>
          <friction>
            <ode>
              <mu>0.1</mu>
              <mu2>0.1</mu2>
              <xacro:if value="${side == 'front_left'}">
                <fdir1 ignition:expressed_in="base_footprint">1 -1 0</fdir1>
              </xacro:if>
              <xacro:if value="${side == 'rear_left'}">
                <fdir1 ignition:expressed_in="base_footprint">1 1 0</fdir1>
              </xacro:if>
              <xacro:if value="${side == 'front_right'}">
                <fdir1 ignition:expressed_in="base_footprint">1 1 0</fdir1>
              </xacro:if>
              <xacro:if value="${side == 'rear_right'}">
                <fdir1 ignition:expressed_in="base_footprint">1 -1 0</fdir1>
              </xacro:if>
              <slip1>0.01</slip1>
              <slip2>0.01</slip2>
            </ode>
          </friction>
          <contact>
            <ode>
              <kp>100.0</kp>
              <kd>100.0</kd>
              <max_vel>1.0</max_vel>
              <min_depth>0.0</min_depth>
            </ode>
          </contact>
        </surface>
      </collision>

2. Load mecanum drive plugin in the xacro file

      <plugin filename="gz-sim-mecanum-drive-system" name="ignition::gazebo::systems::MecanumDrive">

        <!-- wheels -->
        <front_left_joint>wheel_front_left_joint</front_left_joint>
        <front_right_joint>wheel_front_right_joint</front_right_joint>
        <back_left_joint>wheel_rear_left_joint</back_left_joint>
        <back_right_joint>wheel_rear_right_joint</back_right_joint>

        <!-- Technical description -->
        <wheelbase>${wheel_pair_separation}</wheelbase>
        <wheel_separation>${wheel_separation}</wheel_separation>
        <wheel_radius>${wheel_radius}</wheel_radius>
        <odom_publish_frequency>50</odom_publish_frequency>

        <topic>cmd_vel</topic>

      </plugin>

3. Generate the xacro file from robot_state_publisher robot_state_publisher with your xacro file as a parameter.