turtlebot3_state_publisher.launch.py cannot be launched directly: undeclared 'namespace' argument

[mmporong]

Operating System

Ubuntu 22.04 (ROS 2 Humble, ros-humble-turtlebot3-bringup 2.3.6)

Description

turtlebot3_bringup/launch/turtlebot3_state_publisher.launch.py reads
LaunchConfiguration('namespace') but never declares it and gives it no default
value. As a result the file can only be used when it is included from
robot.launch.py, which passes the argument explicitly. Launching it directly
fails before any node starts.

To reproduce

export TURTLEBOT3_MODEL=burger
ros2 launch turtlebot3_bringup turtlebot3_state_publisher.launch.py

urdf_file_name : turtlebot3_burger.urdf
[ERROR] [launch]: Caught exception in launch (see debug for traceback):
launch configuration 'namespace' does not exist

Where it comes from

Introduced in d81e95a
("Add namespace argument for multi robot control in turtlebot3 bringup launch",
2025-02-03).

That commit added both LaunchConfiguration('namespace', default='') and a
matching DeclareLaunchArgument('namespace', ...) to robot.launch.py, but added
only the LaunchConfiguration to turtlebot3_state_publisher.launch.py. The two
files are therefore inconsistent, and the child launch file depends on the parent to
supply the argument.

The affected file is byte-identical on main, jazzy and humble, so all three
branches are affected.

Impact

Any package that includes turtlebot3_state_publisher.launch.py without passing
namespace now fails. For example StanfordASL/asl-tb3-driver
(launch/bringup.launch.py) includes it with no launch arguments.

This may also be related to #1104, which reported a launch failure in this area and
was closed without a code change.

Suggested fix

Apply to turtlebot3_state_publisher.launch.py the same pattern already used in
robot.launch.py: give namespace a default and declare it. This keeps the
multi-robot behaviour intact (empty namespace when the argument is omitted,
namespace:=tb1 still prefixing frames) and additionally makes the argument visible
in ros2 launch ... --show-args.

A pull request with this change is attached.