Resolving Zombie Nodes When Launching Child Launch Files In ROS2

Hey everyone! If you're diving into ROS2 and using launch files to manage your nodes, you might run into a pesky issue: zombie nodes. These are nodes that seem to hang around even after their parent launch file has finished executing, and they can cause some serious headaches. Today, we’re going to break down what causes this, and how you can fix it, especially when you're dealing with parent launch files that start child launch files. Let's get started!

Understanding the Zombie Node Problem

So, what exactly are zombie nodes, and why are they a problem? Imagine you've got a robot with various sensors and actuators, each controlled by a ROS2 node. To simplify things, you might create a parent launch file that starts several child launch files, each responsible for a specific subsystem, like the navigation stack or the perception module. This is a great way to organize your system, but it introduces a potential pitfall.

When a parent launch file starts a child launch file using IncludeLaunchDescription, it essentially spawns a new process to run the child launch file. The parent launch file might finish its execution before the child launch file, especially if the child launch file contains long-running nodes or complex setups. If the nodes in the child launch file aren't properly shut down when the parent process exits, they can become zombie nodes – processes that are still running but are no longer managed by the launch system. These nodes can consume resources, interfere with other processes, and generally make your system unstable.

Why Zombie Nodes Occur

The main reason for zombie nodes is that the signal handling between the parent and child launch processes isn't always perfect. When you terminate the parent launch file (e.g., by pressing Ctrl+C), a signal is sent to the parent process. The parent process is then responsible for propagating this signal to its child processes, including the nodes launched by the child launch file. However, if this signal propagation doesn't happen correctly, or if the nodes in the child launch file don't handle the signal gracefully, you end up with zombie nodes.

Another factor contributing to zombie nodes is the way ROS2 handles node lifetimes. By default, nodes in ROS2 are designed to run until they are explicitly told to stop. If a node doesn't receive a shutdown signal, it will continue running indefinitely, even if the launch file that started it has terminated. This behavior is intentional, as it allows nodes to be long-lived and handle various events over time. However, it also means that you need to be careful about how you manage node lifetimes, especially in the context of parent and child launch files.

Diagnosing Zombie Nodes

Before we dive into the solutions, let's talk about how you can identify zombie nodes. Luckily, ROS2 provides several tools that can help you diagnose this issue.

One of the most straightforward methods is using the ros2 node list command. This command lists all the active nodes in your ROS2 system. If you run this command after terminating a launch file and you see nodes that you expect to be gone, you've likely got zombie nodes.

Another useful tool is ps, the process status command in Linux. You can use ps along with grep to filter processes related to your ROS2 nodes. For example, if you know the name of a node that's supposed to be terminated, you can use ps aux | grep <node_name> to check if it's still running. If the node process is listed in the output, it's a zombie node.

Common Symptoms of Zombie Nodes

Besides using command-line tools, you might notice some common symptoms that indicate the presence of zombie nodes:

• Resource Consumption: Zombie nodes can consume CPU and memory resources, even though they're not actively doing anything useful. This can lead to performance degradation in your system.
• Interference with Other Processes: Zombie nodes might continue to publish or subscribe to topics, which can interfere with other nodes in your system. This can lead to unexpected behavior and make debugging more difficult.
• Failed Shutdowns: When you try to shut down your ROS2 system, you might encounter errors or delays if there are zombie nodes present. The system might wait for these nodes to terminate, which can take a long time or even fail.

Solutions to Prevent Zombie Nodes

Now that we understand the problem and how to diagnose it, let's look at some solutions to prevent zombie nodes when using parent and child launch files in ROS2. There are several techniques you can use, and the best approach often depends on the specific structure of your launch files and nodes.

1. Ensure Proper Signal Handling

The most important step in preventing zombie nodes is to ensure proper signal handling. When the parent launch process receives a termination signal (e.g., SIGINT from Ctrl+C), it needs to propagate this signal to all its child processes, including the nodes launched by the child launch file. Here’s how you can do this:

• Use the shutdown action: ROS2 launch provides a shutdown action that allows you to define actions to be executed when the launch process is terminated. You can use this action to explicitly shut down nodes in the child launch file.
• Utilize the on_exit event: Launch descriptions can include on_exit event handlers that trigger actions when a specific process exits. You can use this to define actions to shut down other nodes or processes that depend on the exiting process.

2. Use the Lifecycle Node Class

ROS2 provides a LifecycleNode class that can help you manage node states and transitions. Lifecycle nodes have a well-defined lifecycle, including states like