Guard DVL bridge spin during ROS shutdown

[yeseorizi]

Summary

Prevent DVLBridge::PostUpdate() from calling rclcpp::spin_some() after the ROS context has been invalidated during shutdown.

Problem

When Gazebo receives SIGINT, the ROS context can be shut down while the simulation update thread is still entering PostUpdate(). The current implementation unconditionally calls rclcpp::spin_some() whenever the simulation is not paused.

In the reproduced failure, spin_some() attempted to construct a temporary executor after the context had become invalid. The resulting rclcpp::exceptions::RCLError was uncaught and aborted the Gazebo server:

terminate called after throwing an instance of 'rclcpp::exceptions::RCLError'
what(): failed to create guard condition: the given context is not valid,
either rcl_init() was not called or rcl_shutdown() was called
...
rclcpp::spin_some(...)
dave_ros_gz_plugins::DVLBridge::PostUpdate(...)
Aborted

Change

• Return early when the simulation is paused, the node is unavailable, or the ROS context is no longer valid.
• Catch the race where the context becomes invalid between the rclcpp::ok() check and executor construction in spin_some().
• Re-throw RCLError when the context is still valid, so unrelated runtime errors are not hidden.

This does not change DVL message conversion or the public interface.

Reproduction

Launch a world containing DVLBridge, confirm DVL messages, and stop the server with SIGINT. Repeat the launch and shutdown several times:

ros2 launch dave_demos dave_sensor.launch.py \
  namespace:=dvl world_name:=new_dvl paused:=false \
  gui:=true headless:=false
# Confirm /dvl/velocity, then press Ctrl-C. Repeat.

The stock bridge reproduced one shutdown abort in three trials. The failing process exited with code 134 from DVLBridge::PostUpdate().

Validation

The exact candidate source in this PR was built and tested in an ARM64 Docker environment with ROS 2 Lyrical and Gazebo Jetty.

Result	Stock bridge	Patched bridge
Trials	3	10
Server ready	3/3	10/10
Gazebo DVL payload	3/3	10/10
ROS DVL payload	2/3	10/10
Advancing simulation stats	3/3	10/10
Clean shutdown	2/3	10/10
Abort / fatal signature	1/3	0/10

Repository checks:

pre-commit run --files gazebo/dave_ros_gz_plugins/src/DVLBridge.cc
# Passed, including clang-format

git diff --check
# Passed

Checklist

• The change is limited to the DVL bridge shutdown race.
• Normal-context exceptions remain visible.
• DVL payload behavior was checked before shutdown.
• Repeated shutdown testing passed with the candidate.
• Repository formatting hooks passed.