xacro (Melodic) parsing content within xml-comment-brackets

[Stefan Kull]

1. Xacro Trouble Report

1.1 Expected behaviour

• xacro processing/interpreter should ignore all content in between xml-comment-brackets <!-- and -->.

1.2 Current behaviour

My analysis:

• Comments/Content in between xml comment brackets, <! -- and -->, are also expanded/interpreted.
  Which causes, in my example, xacro to stop and display error message Undefined substitution argument world .

1.3 Background

• Recently (last month?) my old .world.xacro files stopped working => Undefined substitution argument world .
• I have been using those .xacro files since the summer 2021. Without any changes.
  But now in jan/deb-2022 they have stop working.

1.4 My work environment

• Melodic with the latest updates (Today is: 27 feb, 2022).
  Note! - It looks like it works with Noetic...
• Ubuntu 18.04(64-bit)

2. Examples

See example below "How to repeat the issue", in Melodic, and "Temporary workaround" I made.
(I have cut down the attached examples and ,almost ,only kept the xml comments that causing the error)

2.1 Example: How to repeat the issue

Output from: $ cat TroubleReport_error.xacro
Attached: TroubleReport_error.xacro

<?xml version="1.0" ?> 
<sdf version="1.5"> 

  <!-- 
    ".xacro" => SDF/URDF-macro file that need to be parsed before loading into Gazebo. 

    Example - Via shell/command line: 
      $ rosrun xacro xacro src/Pet-Mk-IV/pet_mk_iv_simulation/worlds/pet_play_yard.world.xacro >  src/Pet-Mk-IV/pet_mk_iv_simulation/worlds/pet_play_yard.world  

    Example - Via launch file: 
      <node name="xacro" pkg="xacro" type="xacro" args="$(find pet_mk_iv_simulation)/worlds/$(arg world).xacro -o $(find pet_mk_iv_simulation)/worlds/$(arg world)" /> 

    http://wiki.ros.org/xacro  
  --> 

  <world name="default"> 
    <!-- REMOVED ALL THE CONTENT IN DEBUGGING PURPOSE... ONLY LEAVING A EMPTY WORLD --> 

    <!-- A global light source --> 
    <include> 
      <uri>model://sun</uri> 
    </include> 

    <!-- A ground plane --> 
    <include> 
      <uri>model://ground_plane</uri> 
    </include> 
  </world> 
</sdf>

Pay attention to the "$(arg world)" in the example above. It is those string, INSIDE the xml-comment-brackets, that causing my problem. See the output/result from xacro below.

Output from: $ rosrun xacro xacro TroubleReport_error.xacro

Undefined substitution argument world 
when processing file: TroubleReport_error.xacro 

2.2 Example: A temporary workaround

As a workaround, I replaced $ with [** dollars **] in between the xml-comment-brackets <!-- and -->.

Output from: $ cat TroubleReport_workaraound.xacro
Attached: TroubleReport_workaraound.xacro

<?xml version="1.0" ?> 
<sdf version="1.5"> 

  <!-- 
    ".xacro" => SDF/URDF-macro file that need to be parsed before loading into Gazebo. 

    Example - Via shell/command line: 
      $ rosrun xacro xacro src/Pet-Mk-IV/pet_mk_iv_simulation/worlds/pet_play_yard.world.xacro >  src/Pet-Mk-IV/pet_mk_iv_simulation/worlds/pet_play_yard.world  

    Example - Via launch file: 
      <node name="xacro" pkg="xacro" type="xacro" args="$(find pet_mk_iv_simulation)/worlds/[**dollar**](arg world).xacro -o $(find pet_mk_iv_simulation)/worlds/[**dollar**](arg world)" /> 

    http://wiki.ros.org/xacro  
  --> 

  <world name="default"> 
    <!-- REMOVED ALL THE CONTENT IN DEBUGGING PURPOSE... ONLY LEAVING A EMPTY WORLD --> 

    <!-- A global light source --> 
    <include> 
      <uri>model://sun</uri> 
    </include> 

    <!-- A ground plane --> 
    <include> 
      <uri>model://ground_plane</uri> 
    </include> 
  </world> 
</sdf>

• Pay attention to how "$(find pet_mk_iv_simulation)", inside the comment, is handled...!
  Expanded to " /home/sk/ros_ws/src/pet_mk_iv_simulation".*
  This should not either happens... But in this case it did not cause any error.

Output from: $ rosrun xacro xacro TroubleReport_workaraound.xacro
Attached: TroubleReport_workaraound.world

<?xml version="1.0" encoding="utf-8"?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from TroubleReport_workaraound.xacro.txt | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<sdf version="1.5">
  <!-- 
    ".xacro" => SDF/URDF-macro file that need to be parsed before loading into Gazebo. 

    Example - Via shell/command line: 
      $ rosrun xacro xacro src/Pet-Mk-IV/pet_mk_iv_simulation/worlds/pet_play_yard.world.xacro >  src/Pet-Mk-IV/pet_mk_iv_simulation/worlds/pet_play_yard.world  

    Example - Via launch file: 
      <node name="xacro" pkg="xacro" type="xacro" args="/home/sk/ros_ws/src/pet_mk_iv_simulation/worlds/[**dollar**](arg world).xacro -o /home/sk/ros_ws/src/pet_mk_iv_simulation/worlds/[**dollar**](arg world)" /> 

    http://wiki.ros.org/xacro  
  -->
  <world name="default">
    <!-- REMOVED ALL THE CONTENT IN DEBUGGING PURPOSE... ONLY LEAVING A EMPTY WORLD -->
    <!-- A global light source -->
    <include>
      <uri>model://sun</uri>
    </include>
    <!-- A ground plane -->
    <include>
      <uri>model://ground_plane</uri>
    </include>
  </world>
</sdf>

2.3 Attached files

Attached: TroubleReport_error.xacro
Attached: TroubleReport_workaraound.xacro
Attached: TroubleReport_workaraound.world

[Robert Haschke]

See #309 (comment) for a fix. Unfortunately, Melodic hasn't seen a release sync for a while. Consider using the debian packages from the ROS testing repo or build xacro from source.

[Robert Haschke]

Closing as Melodic as well as Noetic distros have synced the latest fixes.