Bug Geofence Missing Parameters On Non-Default Target PX4-Autopilot

This article addresses a critical bug encountered in PX4-Autopilot when using geofencing with non-default target configurations. The issue manifests as errors related to missing parameters, specifically affecting builds for custom vehicle targets like spacecraft. This detailed analysis will walk you through the bug's description, reproduction steps, expected behavior, and potential solutions, providing a comprehensive understanding of the problem and its implications for developers and users of PX4. This article serves as a valuable resource for anyone encountering similar issues and offers insights into debugging and resolving parameter-related errors in PX4-Autopilot.

Describe the Bug

The core issue arises when the vehicle is configured with a non-default target, leading to geofencing errors. These errors are characterized by messages indicating that the system cannot retrieve necessary parameters. Specifically, the error logs display messages like ERROR [geofence_breach_avoidance] wrong type passed to param_get() for param (null) and ERROR [parameters] get: param 65535 invalid. These errors are linked to several parameters, including MPC_JERK_MAX, MPC_ACC_HOR, MPC_ACC_HOR_MAX, MPC_JERK_AUTO, MPC_ACC_UP_MAX, and MPC_ACC_DOWN_MAX. These parameters control various aspects of the vehicle's motion, such as maximum jerk, horizontal acceleration, and vertical acceleration limits.

The problem appears to stem from the geofence breach avoidance module attempting to access these parameters even when they are not defined or relevant for the specific target configuration. This is particularly problematic for custom targets, such as spacecraft, that may not require the same motion control parameters as standard drone configurations. The geofencing system is a crucial safety feature in autonomous flight, ensuring that the vehicle operates within predefined boundaries. When these parameters are missing or incorrectly accessed, the geofencing system may not function as intended, potentially leading to unsafe operations. Understanding the root cause of these errors is essential for maintaining the reliability and safety of PX4-based systems.

Further complicating the issue, if one attempts to disable the navigator module (CONFIG_MODULES_NAVIGATOR=n) under the assumption that the target doesn't require it, a compilation error occurs. This error message, error: ‘GF_ACTION’ is not a member of ‘px4::params’, indicates a deeper dependency issue. It suggests that the geofencing system relies on parameters and modules that are seemingly external to its core functionality. This creates a situation where removing seemingly unrelated modules can lead to build failures, highlighting the interconnected nature of the PX4-Autopilot codebase. This interconnectedness makes debugging and resolving issues more challenging, as changes in one part of the system can have unexpected consequences in other areas.

The underlying cause of this behavior points to a potential “spaghetti code” situation, where dependencies between modules are not clearly defined or managed. This lack of clear separation can lead to unexpected errors and makes it difficult to optimize the system for specific target configurations. Addressing this issue requires a thorough review of the codebase to identify and resolve these implicit dependencies. The long-term solution may involve restructuring the code to improve modularity and reduce the reliance of one module on parameters defined in others. This would not only fix the current bug but also make the system more maintainable and extensible in the future.

To Reproduce

To reproduce this bug, follow these steps:

1. Build a spacecraft target, for instance, using the command make px4_sitl_spacecraft gz_atmos with the specified pull request (https://github.com/PX4/PX4-Autopilot/pull/24734).

This command compiles the PX4-Autopilot software for a simulated spacecraft environment using the Gazebo simulator and the atmospheric model. The specified pull request likely contains changes relevant to the spacecraft target configuration, making it essential for reproducing the bug in the intended context.

The make command is a standard build tool used in many software projects, including PX4-Autopilot. It reads a Makefile, which contains instructions on how to compile the code, link the libraries, and create the executable files. The px4_sitl_spacecraft argument specifies the target platform, which in this case is a simulated spacecraft environment. The gz_atmos argument likely refers to a specific Gazebo simulation environment that includes atmospheric effects, which are relevant for spacecraft simulations.

By building the software with this command, you create an environment where the bug is likely to manifest. This allows you to observe the error messages described in the bug report and investigate the underlying cause. The ability to reproduce a bug is a critical step in the debugging process, as it allows developers to systematically test different solutions and verify that they effectively resolve the issue. In this case, reproducing the bug involves setting up the specific build environment and running the software, which will then trigger the error messages related to the missing geofence parameters. This provides a concrete starting point for further investigation and resolution.

Expected Behavior

The expected behavior is that if modules depend on parameters defined outside of them, the system should allow for those modules to be excluded from compilation when their dependencies are not required. This principle of modularity and conditional compilation is crucial for optimizing the system for various target configurations.

In an ideal scenario, if a module, such as the geofencing module, relies on specific parameters that are only relevant for certain vehicle types or configurations, the build system should be able to detect this dependency and exclude the module from compilation if those parameters are not defined or used. This would not only prevent the error messages related to missing parameters but also reduce the overall size and complexity of the compiled software, making it more efficient and easier to maintain.

For example, a spacecraft configuration may not require the same motion control parameters as a standard quadrotor drone. If the geofencing module attempts to access these parameters, it should ideally be possible to exclude the module from the build process, preventing the errors described in the bug report. This requires a robust dependency management system that can track the relationships between modules and parameters and make informed decisions about which modules to include or exclude during compilation.

The current behavior, where the system attempts to access parameters even when they are not defined, indicates a lack of proper dependency management. This can lead to unexpected errors and makes it difficult to customize the system for specific use cases. The desired behavior is a more modular and flexible system where modules can be selectively included or excluded based on their dependencies, allowing for a more tailored and efficient build process. This would not only resolve the current bug but also improve the overall maintainability and extensibility of the PX4-Autopilot software.

Software Version

This bug was observed in a SITL (Software In The Loop) simulation environment, which is a crucial testing platform for PX4-Autopilot. SITL allows developers to test the software in a simulated environment before deploying it to real hardware, reducing the risk of errors and crashes in the field.

The fact that the bug was observed in SITL indicates that it is likely a software-related issue rather than a hardware-specific problem. This means that the bug can be addressed by modifying the software code and recompiling the system. SITL provides a controlled and repeatable environment for testing these modifications and verifying that they effectively resolve the issue.

Additional Information

This section includes other details about the bug, such as flight logs, screenshots, and specifics about the vehicle setup. While the original report indicates that there are no flight logs or screenshots available, the information provided about the vehicle type (Other, specifically spacecraft) and the absence of details about component wiring suggests that this is a highly customized setup. This further emphasizes the importance of modularity and conditional compilation, as custom setups are more likely to encounter issues related to missing or irrelevant parameters.

Solution Approaches

Addressing this bug requires a multi-faceted approach, focusing on improving dependency management and modularity within the PX4-Autopilot codebase. Here are some potential solutions:

1. Conditional Parameter Access: Implement checks within the geofencing module to ensure that parameters are only accessed if they are defined and relevant for the current target configuration. This can involve using conditional statements to check for the existence of specific parameters before attempting to read their values. This approach would prevent the wrong type passed to param_get() errors by avoiding the attempt to access non-existent parameters.

2. Dependency Management: Refactor the codebase to explicitly define dependencies between modules and parameters. This can involve creating a dependency graph that maps the relationships between different components of the system. This graph can then be used during the build process to determine which modules and parameters are required for a specific target configuration. This would allow the system to automatically exclude modules that are not needed, reducing the size and complexity of the compiled software.

3. Modularization: Further modularize the geofencing system to separate core functionality from target-specific implementations. This can involve creating abstract interfaces for motion control parameters and providing concrete implementations for different vehicle types. This would allow the geofencing system to operate independently of the specific vehicle configuration, reducing the risk of errors related to missing parameters.

4. Build System Improvements: Enhance the build system to support conditional compilation based on target configuration and parameter availability. This can involve adding new options to the make command or using a more advanced build system that can automatically resolve dependencies and exclude unnecessary modules. This would provide a more flexible and efficient way to build the PX4-Autopilot software for different target platforms.

5. Error Handling: Improve error handling within the geofencing module to provide more informative error messages when parameters are missing. This can involve logging the specific parameter that is missing and providing guidance on how to resolve the issue. This would make it easier for developers to diagnose and fix parameter-related errors.

By implementing these solutions, the PX4-Autopilot project can address the current bug and improve the overall robustness and maintainability of the software. This will benefit developers and users by reducing the risk of errors and making it easier to customize the system for specific use cases.

Conclusion

The geofencing bug described in this article highlights the importance of proper dependency management and modularity in complex software systems like PX4-Autopilot. By addressing the root cause of this bug and implementing the proposed solutions, the PX4-Autopilot project can enhance its reliability, flexibility, and maintainability. This will ultimately lead to a more robust and user-friendly platform for autonomous flight, benefiting developers, researchers, and end-users alike. The key takeaways from this analysis include the need for clear dependency definitions, conditional parameter access, modular design, and robust error handling. By focusing on these areas, the PX4-Autopilot community can continue to improve the quality and capabilities of this open-source flight control system.