[ROS2] Loading plugins from a non-ROS package

[schornakj]

In ROS 1, the paths to all packages used in the project workspace are added to the ROS_PACKAGE_PATH environment variable, including pure CMake packages. This allows pluginlib to find libraries defined in non-ROS libraries, since the ROS 1 implementation of ClassLoader uses ros::package::path().

However, in ROS 2 there are different environment variables for CMAKE_PREFIX_PATH and AMENT_PREFIX_PATH. The first one contains all CMake packages used in my project, while the second one is a subset of the first only containing ament CMake packages. The ROS 2 implementation of ClassLoader uses ament_index_cpp::get_package_prefix(), which apparently only looks for packages in AMENT_PREFIX_PATH. This means that (so far) I can no longer load libraries defined in non-ROS packages.

The Tesseract motion planning pipeline is an example of something affected by this difference in behavior. The tesseract_plugins package defines several plugins which use libraries created by the ROS-agnostic tesseract_collision package, and these plugins are loaded at runtime by classes in the tesseract_monitoring ROS package. This works just fine in ROS 1 but in my ROS 2 port I get errors like:

[my_project_planning_manager_node-1] terminate called after throwing an instance of 'pluginlib::ClassLoaderException'
[my_project_planning_manager_node-1]   what():  package 'tesseract_collision' not found, searching: [/home/joe/workspaces/ros2_ws/install/xacro, /home/joe/workspaces/ros2_ws/install/opencv_tests, /home/joe/workspaces/ros2_ws/install/joint_state_publisher, /home/joe/workspaces/ros2_ws/install/vision_opencv, /home/joe/workspaces/ros2_ws/install/my_project_planning, /home/joe/workspaces/ros2_ws/install/tesseract_monitoring, /home/joe/workspaces/ros2_ws/install/tesseract_rosutils, /home/joe/workspaces/ros2_ws/install/tesseract_plugins, /home/joe/workspaces/ros2_ws/install/tesseract_msgs, /home/joe/workspaces/ros2_ws/install/srdfdom, /home/joe/workspaces/ros2_ws/install/robot_state_publisher, /home/joe/workspaces/ros2_ws/install/rct_ros_tools, /home/joe/workspaces/ros2_ws/install/rct_image_tools, /home/joe/workspaces/ros2_ws/install/rct_optimizations, /home/joe/workspaces/ros2_ws/install/octomap_msgs, /home/joe/workspaces/ros2_ws/install/mutable_transform_publisher, /home/joe/workspaces/ros2_ws/install/mutable_transform_publisher_msgs, /home/joe/workspaces/ros2_ws/install/image_geometry, /home/joe/workspaces/ros2_ws/install/cv_bridge, /opt/ros/dashing]

Any thoughts? I'd be open to package configuration workarounds on my end as well.

(pinging @Levi-Armstrong as the Tesseract package maintainer)

[schornakj]

A slightly messy workaround is to manually append the non-ROS package containing the libraries I want to load to the AMENT_PREFIX_PATH environment variable, since ament_index_cpp::get_resource() uses this variable to populate its search paths (via ament_index_cpp::get_search_paths()).

edit: It also looks for an empty file with the same name as the package at a specific path within the package install directory: ros2_ws/install/tesseract_collision/share/ament_index/resource_index/packages/tesseract_collision