expose native participant

[dirk-thomas]

This is an example how to expose native handles from a rmw implementation. It only exposes the eprosima::fastrtps::Participant but other handles could be supported in a similar fashion. The patch demonstrate how this is possible and that it can be achieved by each rmw implementation without requiring any changes in the rmw interface.

When the function is being used with a different rmw implementation at runtime get_participant returns NULL.

In order to try this feature I modified the talker in the demo_nodes_cpp package. The changes are straight forward. Only getting from the C++ Node to the rmw_node_t is not "very pretty":

• package.xml

<depend>rmw_fastrtps_cpp</depend>

• CMakeLists.txt

find_package(rmw_fastrtps_cpp REQUIRED)
...
ament_target_dependencies(talker "rmw_fastrtps_cpp")

• talker.cpp

#include "rmw_fastrtps_cpp/internal/get_participant.hpp"
...
// auto node = rclcpp::node::Node::make_shared("talker");
...
rcl_node_t * rcl_node = node->get_node_base_interface()->get_rcl_node_handle();
rmw_node_t * rmw_node = rcl_node_get_rmw_handle(rcl_node);
eprosima::fastrtps::Participant * p = rmw_fastrtps_cpp::get_participant(rmw_node);

[wjwwood]

+1, makes sense to me.

I always imagined the API added here would be in a separate package, but I can't think of a reason it needs to be at this point.

[dirk-thomas]

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