Improve Rtest: Time Advancement For Timer Testing

by Benjamin Cohen 50 views

Hey everyone! 👋 Today, I want to discuss a feature proposal aimed at improving how we handle testing timers within the rtest framework. Specifically, I'm suggesting a shift towards using time advancement as a trigger for timers, which I believe will make our tests more robust and less reliant on implementation details. Let's dive into the proposal and see what you guys think!

The Current Approach and Its Challenges

Currently, the way rtest handles testing timers feels a bit fragile. It often requires a deep understanding of the underlying implementation, which can lead to tests that are brittle and prone to breaking when the implementation changes. This tight coupling between tests and implementation details isn't ideal, as it makes refactoring and maintaining the codebase more challenging. We want our tests to verify behavior, not internal mechanisms, right?

The core issue is that the current approach often involves directly manipulating or inspecting the internal state of the timer mechanism. This means our tests are essentially poking around inside the black box, instead of observing its external behavior. This can lead to tests that pass when the internal implementation is a certain way, but fail when it's changed, even if the external behavior remains the same. This is a classic example of tests that are too tightly coupled to the implementation.

Imagine this scenario: You're working on a feature that involves timers. You write a test that directly checks the timer's internal state to ensure it's firing correctly. Everything works great, and you move on. Later, someone refactors the timer implementation to improve performance or add new features. However, because your test was tightly coupled to the old implementation, it now fails, even though the timer's external behavior—the thing you actually care about—is still correct. This is frustrating and time-consuming, and it's exactly what we want to avoid.

We want our tests to be more resilient to changes in the underlying implementation. We want them to focus on the behavior of the timer, not the details of how it's implemented. This is where the idea of time advancement comes in.

The Proposed Solution: Time Advancement as a Trigger

My proposal is to use time advancement as a trigger for timers during testing. Instead of directly manipulating the timer's internal state, we would simulate the passage of time and let the timer react accordingly. This approach would allow us to test the timer's behavior in a more realistic and less intrusive way. Think of it as fast-forwarding through the timeline to see how the timer behaves over time. This makes the test more robust and closer to real-world scenarios.

The core idea is to have a TestClock class that allows us to control the flow of time within our tests. This clock would be used to simulate the passage of time, triggering timers as they would be in a real-world scenario. By advancing the clock, we can effectively fast-forward through time, allowing us to test timers that fire at specific intervals or after certain conditions are met.

This approach has several advantages. First, it makes our tests less dependent on the internal implementation of the timer mechanism. Second, it allows us to test timers in a more realistic way, as we're simulating the actual passage of time. Third, it makes our tests easier to understand and maintain, as they're focused on the behavior of the timer, not the details of its implementation. By simulating time, we isolate the timer's behavior and test it independently from other parts of the system.

Code Snippet: A Glimpse at the TestClock

To give you a better idea of what I'm proposing, here's a snippet of what a TestClock class might look like:

class TestClock
{
public:
  TestClock(rclcpp::Node::SharedPtr node) : timers_{findTimers(node)}
  {....}

   void advance(std::chrono::milliseconds time)
    {

        for (std::chrono::milliseconds::rep rep{0}; rep < time.count(); ++rep)  {
            now_ += std::chrono::nanoseconds(std::chrono::milliseconds(1)).count();
            if (rcl_set_ros_time_override(clock_, now_) != RCL_RET_OK) {
                throw std::runtime_error{"TestClock::advanceMs() error"};
            }
            for(auto& timer : timers_) {
                auto data = timer_->call();
                if(data) {
                  timer_->execute_callback(data);
                }
            }
        }
    }

  void advanceMs(int64_t milliseconds) { advance(std::chrono::milliseconds(milliseconds)); }
  void resetClock(const rcl_time_point_value_t tv = 0L) {...}
private:
  rcl_clock_t * clock_{nullptr};
  rcl_time_point_value_t now_{0L};
  std::vector<std::shared_ptr<rclcpp::TimerBase>> timers_;

};

In this snippet, the TestClock class has methods for advancing time by a specified duration (advance) and resetting the clock to a specific time (resetClock). The advance method iterates through the specified time, incrementing the clock's internal time and triggering any timers that are due to fire. This allows us to simulate the passage of time and test the behavior of timers in a controlled environment. This method effectively creates a mini-timeline within our test, allowing us to fast-forward and observe timer behavior.

Key Components of the TestClock

Let's break down the key components of this TestClock implementation:

  • TestClock(rclcpp::Node::SharedPtr node): The constructor takes a shared pointer to an rclcpp::Node. This node is used to find all the timers associated with that node, which are then stored in the timers_ member variable. This ensures that the TestClock is aware of all the timers that need to be managed during the test.
  • void advance(std::chrono::milliseconds time): This is the core method for advancing time. It takes a std::chrono::milliseconds object as input, representing the amount of time to advance. The method then iterates through each millisecond of the specified time, updating the internal clock (now_) and triggering any timers that are due to fire. Inside the loop, rcl_set_ros_time_override is called to override the ROS time with the new time. This is crucial for simulating time in a ROS environment. The method also iterates through the timers_ vector, calling the call method on each timer. If the timer is due to fire, its callback is executed.
  • void advanceMs(int64_t milliseconds): This is a convenience method that simplifies advancing time by a specified number of milliseconds. It simply calls the advance method with the appropriate std::chrono::milliseconds object.
  • void resetClock(const rcl_time_point_value_t tv = 0L): This method resets the clock to a specific time. The default value is 0, which corresponds to the Unix epoch. This is useful for ensuring that tests start from a known state.
  • rcl_clock_t * clock_{nullptr}: This member variable stores a pointer to the ROS clock. This clock is used to override the ROS time during testing.
  • rcl_time_point_value_t now_{0L}: This member variable stores the current time of the clock, represented as a ROS time point value.
  • std::vector<std::shared_ptr<rclcpp::TimerBase>> timers_: This member variable stores a vector of shared pointers to the timers associated with the node. This allows the TestClock to iterate through the timers and trigger them as needed.

This TestClock class provides a powerful and flexible way to simulate time in our tests, allowing us to test timers in a more realistic and less intrusive way. By controlling the flow of time, we can ensure that our timers behave as expected under various conditions.

Benefits of Time Advancement

Adopting time advancement as a testing strategy offers several significant benefits:

  • Reduced Fragility: Tests become less susceptible to breaking due to internal implementation changes. Since we're testing the timer's behavior from an external perspective, changes to the internal workings of the timer are less likely to affect our tests. This means we can refactor and optimize the timer implementation without fear of breaking our tests.
  • Improved Realism: Simulating time provides a more accurate representation of how timers behave in a real-world scenario. Instead of directly manipulating the timer's state, we're allowing it to react to the passage of time, just as it would in a production environment. This makes our tests more realistic and provides greater confidence in the timer's behavior.
  • Enhanced Testability: Complex timer scenarios, such as those involving multiple timers or timers that interact with other components, become easier to test. By controlling the flow of time, we can orchestrate complex scenarios and verify that the timers behave as expected under various conditions. We can fast-forward through time, pause it, and rewind it, allowing us to thoroughly test even the most intricate timer interactions.
  • Simplified Maintenance: Tests become easier to understand and maintain, as they focus on the timer's behavior rather than its implementation details. This makes it easier for developers to understand the purpose of the tests and to modify them as needed. Cleaner tests lead to a more maintainable codebase.

By focusing on the timer's external behavior, we create tests that are more robust, realistic, and maintainable. This ultimately leads to a more reliable and easier-to-maintain codebase.

POC (Proof of Concept) and Next Steps

The code snippet I shared earlier gives you a glimpse into how a POC for this feature might look. The TestClock class is the heart of the POC, providing the ability to advance time and trigger timers. This POC would serve as a foundation for further development and refinement.

Next Steps

If the library maintainers agree that this is a valuable feature, I'm excited to volunteer my time to work on it! Here are the next steps I envision:

  1. Gather Feedback: I'd love to hear your thoughts and feedback on this proposal. What do you think of the idea of using time advancement for testing timers? Are there any potential drawbacks or challenges that you foresee?
  2. Refine the Design: Based on the feedback I receive, I'll refine the design of the TestClock class and the overall approach. This might involve exploring different ways to advance time, handle timer callbacks, or integrate with the existing rtest framework.
  3. Implement a Working Prototype: I'll then implement a working prototype of the feature, including unit tests to ensure it functions correctly. This prototype will serve as a proof of concept and a foundation for further development.
  4. Integrate with rtest: Finally, I'll work on integrating the feature into the rtest framework, making it available for use in real-world testing scenarios. This will involve modifying the rtest API and adding documentation to guide users on how to use the new feature.

Call to Action

I believe this feature would be a significant improvement to the rtest framework, making our tests more robust and easier to maintain. I'm eager to hear your thoughts and get your feedback on this proposal. Let's discuss how we can make testing timers in rtest even better! What are your initial thoughts, guys? Any potential roadblocks or alternative approaches we should consider?

Conclusion

In conclusion, the proposal to use time advancement for testing timers in rtest offers a promising approach to improve the robustness and maintainability of our tests. By shifting our focus from internal implementation details to external behavior, we can create tests that are less fragile and more realistic. The TestClock class provides a mechanism for simulating time and triggering timers, allowing us to test complex timer scenarios in a controlled environment. If the library maintainers agree, I'm excited to contribute to this feature and help make rtest an even more powerful testing tool. Let's work together to make our testing process more efficient and reliable!