1. When creating a publisher/subscriber/service/client/etc., appropriate references should be kept in order to correlate with other tracepoints related to the same instance.
1. When creating a publisher/subscription, the effective topic name should be included (i.e. including namespace and after remapping).
2. When publishing a message, some sort of message identifier should be included in the tracepoint so it can be tracked through DDS up to the subscriber's side.
To make tracing ROS 2 more accessible and easier to adopt, we can put effort into integrating LTTng session setup & recording into the ROS 2 launch system.
This might include converting existing `tracetools` scripts to more flexible Python scripts, and then plugging that into the launch system.
This section includes information about ROS 2's design & architecture through descriptions of the main execution flows. The instrumentation can then be built around that.
In the call to `rclcpp::init()`, a process-specific `rclcpp::Context` object is fetched and CLI arguments are parsed. Much of the work is actually done by `rcl` through a call to `rcl_init()`. This call processes the `rcl_context_t` handle, which is wrapped by the `Context` object. Also, inside this call, `rcl` calls `rmw_init()` to process the `rmw` context (`rmw_context_t`) as well. This `rmw` handle is itself part of the `rcl_context_t` handle.
In ROS 2, a process can contain multiple nodes. These are sometimes referred to as "components."
These components are instanciated by the containing process. They are usually classes that extend `rclcpp::Node`, so that the node initialization work is done by the parent constructor.
This parent constructor will allocate its own `rcl_node_t` handle and call `rcl_node_init()`, which will validate the node name/namespace. `rcl` will also call `rmw_create_node()` to get the node's `rmw` handle (`rmw_node_t`). This will be used later by publishers and subscriptions.
The component calls `create_publisher()`, a `rclcpp::Node` method for convenience. That ends up creating an `rclcpp::Publisher` object which extends `rclcpp::PublisherBase`. The latter allocates an `rcl_publisher_t` handle, fetches the corresponding `rcl_node_t` handle, and calls `rcl_publisher_init()` in its constructor. `rcl` does topic name expansion/remapping/validation. It creates an `rmw_publisher_t` handle by calling `rmw_create_publisher()` of the given `rmw` implementation and associates with the node's `rmw_node_t` handle and the publisher's `rcl_publisher_t` handle.
If intra-process publishing/subscription is enabled, it will be set up after creating the publisher object, through a call to `PublisherBase::setup_intra_process()`, which calls `rcl_publisher_init()`.
Subscription creation is done in a very similar manner.
The componenent calls `create_publisher()`, which ends up creating an `rclcpp::Subscription` object which extends `rclcpp::SubscriptionBase`. The latter allocates an `rcl_subscription_t` handle, fetches its `rcl_node_t` handle, and calls `rcl_subscription_init()` in its constructor. `rcl` does topic name expansion/remapping/validation. It creates an `rmw_subscription_t` handle by calling `rmw_create_subscription()` of the given `rmw` implementation and associates it with the node's `rmw_node_t` handle and the subscription's `rcl_subscription_t` handle.
If intra-process publishing/subscription is enabled, it will be set up after creating the subscription object, through a call to `Subscription::setup_intra_process()`, which calls `rcl_subscription_init()`. This is very similar to a normal (inter-process) subscription, but it sets some flags for later.
An `rclcpp::executor::Executor` object is created for a given process. It can be a `SingleThreadedExecutor` or a `MultiThreadedExecutor`.
Components are instanciated, usually as a `shared_ptr` through `std::make_shared<Component>()`, then added to the executor with `Executor::add_node()`.
After all the components have been added, `Executor::spin()` is called. `SingleThreadedExecutor::spin()` simply loops forever until the process' context isn't valid anymore. It fetches the next `rclcpp::AnyExecutable` (e.g. subscription, timer, service, client), and calls `Executor::execute_any_executable()` with it. This then calls the relevant `execute*()` method (e.g. `execute_timer()`, `execute_subscription()`, `execute_intra_process_subscription()`, `execute_service()`, `execute_client()`).
Subscriptions are handled in the `rclcpp` layer. Callbacks are wrapped by an `rclcpp::AnySubscriptionCallback` object, which is registered when creating the `rclcpp::Subscription` object.
In `execute_*subscription()`, the `Executor` asks the `Subscription` to allocate a message though `Subscription::create_message()`. It then calls `rcl_take*()`, which calls `rmw_take_with_info()`. If that is successful, the `Executor` then passes that on to the subscription through `rclcpp::SubscriptionBase::handle_message()`. This checks if it's the right type of subscription (i.e. inter vs. intra process), then it calls `dispatch()` on the `rclcpp::AnySubscriptionCallback` object with the message (cast to the actual type). This calls the actual `std::function` with the right signature.
To publish a message, an object is first allocated and then populated by the `Component` (or equivalent). Then, the message is sent to the `Publisher` through `publish()`. This then passes that on to `rcl`, which itself passes it to `rmw`.
Service server creation is similar to subscription creation. The `Component` calls `create_service()` which ends up creating a `rclcpp::Service`. In its constructor, it allocates a `rcl_service_t` handle, then calls `rcl_service_init()`. This processes the handle and validates the service name. It calls `rmw_create_service()` to get the corresponding `rmw_service_t` handle.
Service callbacks are similar to subscription callbacks. In `execute_service()`, the `Executor` allocates request header and request objects. It then calls `rcl_take_request()` and passes them along with the service handle.
`rcl` calls `rmw_take_request()`. If those are successful, then the `Executor` calls `handle_request()` on the `Service`. This casts the request to its actual type, allocates a response object, and calls `dispatch()` on its `AnyServiceCallback` object, which calls the actual `std::function` with the right signature.
For the service response, `Service` calls `rcl_send_response()` which calls `rmw_send_response()`.
Client creation is similar to publisher creation. The `Component` calls `create_client()` which ends up creating a `rclcpp::Client`. In its constructor, it allocates a `rcl_client_t` handle, then calls `rcl_client_init()`. This validates and processes the handle. It also calls `rmw_create_client()` which creates the `rmw_client_t` handle.
A client request has multiple steps. The `Component` (or the owner of the `Client`) first creates a request object. It then calls `Client::async_send_request()` with the request. It can also provide a callback, but it's optional. The `Client` passes that on to `rcl` by calling `rcl_send_request()`. `rcl` generates a sequence number and assigns it to the request, then calls `rmw_send_request()`. Once this is done, the `Client` puts this sequence number in an internal map along with the created promise and future objects, and the callback (which might simply be empty).
At this point, the `Client` could simply let its callback be called. It can also use the future object returned by `async_send_request()`, and call `rclcpp::spin_until_future_complete()`. This waits until the future object is ready, or until timeout, and returns.
If this last call was successful, then the `Component` can get the result and do something with it.
Timer creation is similar to subscription creation. The `Component` calls `create_service()` which ends up creating a `rclcpp::WallTimer`. In its constructor, it creates a `rclcpp::Clock` object, which (for a `WallTimer`) is simply a nanosecond clock. It then allocates a `rcl_timer_t` handle, then calls `rcl_timer_init()`. This processes the handle and validates the period.
Note that `rcl_timer_init()` can take a callback as a parameter, but right now that feature is not used anywhere (`nullptr` is given), and callbacks are instead handled in the `rclcpp` layer.
Timer callbacks are similar to susbcription callbacks. In `execute_timer()`, the `Executor` calls `execute_callback()` on the `WallTimer`. The timer then calls `rcl_timer_call()` with its `rcl_timer_t` handle and checks if the callback should be called.
If it that is the case, then the timer will call the actual `std::function`. Depending on the `std::function` that was given when creating the timer, it will either call the callback without any parameters or it will pass a reference of itself.
```mermaid
sequenceDiagram
participant Executor
participant WallTimer
participant rcl
participant tracetools
Note over Executor: execute_timer()
Executor->>WallTimer: execute_callback()
WallTimer->>rcl: rcl_timer_call(rcl_timer_t) : ret
Note over rcl: validates and updates timer
opt RCL_RET_TIMER_CANCELED != ret && RCL_RET_OK == ret
This includes transposing the existing ROS 1 instrumentation to ROS 2, wherever applicable. This step may not include instrumenting DDS implementations, and thus may be limited to the layer(s) right before `rmw`.
ROS offer a client library (`rcl`) written in C as the base for any language-specific implementation, such as `rclcpp` and `rclpy`.
However, `rcl` is obviously fairly basic, and still does leave a fair amount of implementation work up to the client libraries. For example, callbacks are not handled in `rcl`, and are left to the client library implementations.
This means that some instrumentation work will have to be re-done for every client library that we want to trace. We cannot simply instrument `rcl`, nor can we only instrument the base `rmw` interface if we want to dig into that.
This effort should first focus on `rcl` and `rclcpp` , but `rclpy` should eventually be added and supported.
### ROS 1/2 compatibility
We could look into making analyses work on both ROS 1 and ROS 2, through a common instrumentation interface (or other abstraction).
