Incremental mutation testing in the Bitcoin Core

In the context of Bitcoin Core, a weekly mutation testing cycle is conducted on the master branch, where mutants are generated and assessed through unit, functional, and fuzz testing. This process is both time-consuming and computationally expensive due to the need for code compilation and extensive test execution, particularly for functional tests which can significantly extend the duration of analysis.

The introduction of incremental mutation testing has been explored to mitigate the time and resource constraints associated with traditional mutation testing approaches. By focusing only on modified parts of the code since the last analysis, and leveraging caching for unchanged components, incremental mutation testing substantially reduces the execution time, making it more feasible for continuous integration workflows. This approach has been trialed on personal servers, selectively targeting PRs to assess the impact of recent changes without retesting the entire codebase.

Despite its benefits, mutation testing faces challenges, notably the generation of equivalent and unproductive mutants. Equivalent mutants, being indistinguishable from the original code in their behavior, and unproductive mutants, which do not contribute meaningful insights into the test suite's effectiveness, both require significant manual effort to identify and exclude from analysis. Drawing from practices at Google, feedback mechanisms and strategic avoidance of certain mutations have proven effective in generating more relevant mutants.

The application of mutation testing has extended beyond traditional unit and functional tests to evaluate fuzz targets within Bitcoin Core. Contrary to previous perceptions, studies have shown that well-designed fuzz targets can achieve high mutation scores, indicating their effectiveness in detecting logical bugs. This revelation underscores the importance of enhancing fuzz targets with assertions and metamorphic relations to improve their bug detection capabilities.

In conclusion, mutation testing, especially when applied incrementally and focused on the most relevant mutations, offers valuable insights into the robustness of test suites and the detection of subtle faults in software systems. For Bitcoin Core contributors, ongoing participation and feedback in mutation testing efforts are essential for refining the process and ensuring the continued improvement of test coverage and effectiveness. Contributors are encouraged to engage with mutation testing analyses, provide feedback on reported mutants, and utilize resources like corecheck.dev/mutation for comprehensive mutation analysis reports.