Robotics - ROS2 Behavior Tree Cpp

Basic Constructs

• BT ports: BehaviorTree-CPP needs a compile-time contract for every node type, a list of ports (name + type + optional default + description).
  • e.g., BT::InputPort<std::string>("twist_topic")
  • • When the XML loader meets a tag such as: <Action ID="FixedStandoffOrbit" point_cloud_topic="/points" …/>
  • If the name exists and the string in XML can be converted to the declared C++ type (double, int, bool, std::string, custom structs…), the attribute becomes the runtime value of that port.
  • Inside your node you call getInput<T>("port_name"). That function pulls the string from the blackboard, converts it to T and returns it as a BT::Expected<T> (so you can handle missing / malformed data).

Nodes

• A CoroAction is to pause, send an action goal, wait, get an action result, resume.
• destructor order:
  1. the dtor body 2.members in a reverse order

Quirks & Known Bugs

Do NOT put a tf_listener in init() (Till ROS Humble)

This is not a BT bug, yet a bug in TransformListener. The call chain (on the same thread) is:

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FixedStandoffOrbit::init() -> initialize a tf_listener

The issue is that an initialized tf_listener may cause a hanging issue if destruction comes too soon. Here is an issue report detailing this issue. Here’s why in summary:

Currently, the tf_listener creates its own SingleThreadedExecutor and spin the executor on a separate thread.

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auto executor = std::make_shared<rclcpp::executors::SingleThreadedExecutor>();
auto run_func =
[executor](rclcpp::node_interfaces::NodeBaseInterface::SharedPtr node_base_interface) {
    executor->add_node(node_base_interface);
    executor->spin();
    executor->remove_node(node_base_interface);
};
dedicated_listener_thread_ = thread_ptr(
new std::thread(run_func, node_base_interface),
[executor](std::thread * t) {
    executor->cancel();
    t->join();
    delete t;
});

In the case where the dedicated_listener_thread_ does not get to start before main calls the destructor (and tries to join the thread), the destructor hangs because dedicated_listener_thread_ is still running.

1. main thread creates dedicated_listener_thread_
2. main thread perform all its jobs and TransformListener destructor
3. Main thread in TransformListener destructor calls executor->cancel() which sets executor.spinning = false
4. Main thread waits for dedicated_listener_thread_ to finish
5. Dedicated thread starts running and calls executor->spin()
6. Dedicated thread at the beginning of executor->spin() sets executor.spinning = true
7. Dedicated thread is spinning waiting for messages or cancel which will not happen because main thread is blocked in t->join();

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