rclc_support_init freezes microros while not running the agent

[N-K1998]

Issue template

• Hardware description: STM
• RTOS: freeRTOS
• Installation type: modules
• Version or commit hash: rolling

I am trying to define a custom transport for microros and I have encountered a problem. After defining the custom callbacks and all that is needed the rclc_support_init freezes the program.

Does rclc_support_init function need an agent running to work properly?

[pablogs9]

Yes, normally rclc_support_init needs the micro-ROS Agent, if your use case does not guarantee the presence of the Agent, take a look at the reconnection example: https://docs.vulcanexus.org/en/latest/rst/tutorials/micro/handle_reconnections/handle_reconnections.html

[N-K1998]

Ok, so I need to implement one. Thank you.

[pablogs9]

Please report if this solves the issue and close on this case

[N-K1998]

Yes, ofc. Is it normal that the program does not give an error but it literally stops?

[pablogs9]

Well normally rclc_support_init tries to connect multiple times before returning. By default 10 times and up to 1 second per try: https://github.com/eProsima/Micro-XRCE-DDS-Client/blob/97175304425c5bee87c6fddd99de1ef8d0c394dc/CMakeLists.txt#L57-L58

So it would be normal that without an agent rclc_support_init takes up to 10 seconds. If it takes more, maybe you are having a hard fault in the execution.

[N-K1998]

Do the custom transport functions run on separate tasks? (Microros running on freeRTOS) I mean the one that you can specify in rmw_uros_set_custom_transport.

[pablogs9]

No, micro-ROS do not spawn any thread/task, every API call will run exclusively in the thread it is called.

[N-K1998]

In this documentation https://www.freertos.org/2020/09/micro-ros-on-freertos.html it states that: "Using the FreeRTOS scheduler, micro-ROS is able to manage its main task and the priorities of the tasks in charge of the transport layers. Usually, tasks in charge of networking stack or serial interfaces have to be preempted over the micro-ROS app". I supposed there were tasks for executing custom transport code, for example, to execute the custom_read loop.

[pablogs9]

No, there aren't.

At some transport implementations, maybe the network stack has a task for handling incoming/outcoming packets, but no task is created by the micro-ROS stack itself.

On the other hand, micro-ROS API can be used for different tasks at the user application level, but thread-safe operation shall be enabled.

[N-K1998]

Considering a FreeRTOS task where I call rmw_uros_set_custom_transport and then I put the application code (publish/subscribe). How the application code will be able to execute if the custom read executes in a loop in the same task? Or maybe, the solution is to reserve a task for the microros init code and another for executing the microros application.

[pablogs9]

Because the micro-ROS stack, reads from your custom transport callback in the executor spin. If you debug the application, you will see that under the hood, the executor spin will end here:

https://github.com/micro-ROS/rmw_microxrcedds/blob/bc4eb312ac4601a4137c35f4a56b9b83b4b18339/rmw_microxrcedds_c/src/rmw_wait.c#L108

And under the hood, the middleware will call your read transport operation.