KSP 2.3.x Coverage Drop With Kotlinx-serialization Data Classes

Have you noticed a drop in your code coverage after upgrading to KSP 2.3.x, especially when using kotlinx-serialization with data classes? You're not alone! This article delves into a peculiar bug affecting coverage calculation in KSP (Kotlin Symbol Processing) versions 2.3.2 and later, specifically when dealing with data classes annotated with kotlinx-serialization attributes. We'll explore the issue, its impact, and potential solutions.

Understanding the Coverage Drop in KSP 2.3.x

Code coverage is a crucial metric for gauging the thoroughness of your testing efforts. It indicates the percentage of your codebase that is executed when running your tests. A significant drop in coverage, like the one observed with KSP 2.3.x, can raise concerns about the reliability of your tests and the potential for undetected bugs. When you see a noticeable drop, like the one many developers are experiencing with KSP 2.3.x, it's crucial to investigate the root cause promptly. This not only helps maintain the integrity of your code but also ensures that your testing efforts accurately reflect the robustness of your application. Identifying the cause early on can prevent potential issues from escalating and causing more significant problems down the line. Therefore, understanding the reasons behind a drop in code coverage is a key aspect of quality assurance and maintaining a healthy development lifecycle.

In the specific scenario we're discussing, the problem manifests when using kotlinx-serialization annotations on data classes. With earlier versions of KSP, such as 2.0.1, developers were achieving 100% coverage for these classes. However, after upgrading to version 2.3.2, coverage mysteriously dropped to 50%. This discrepancy is particularly perplexing because the annotations themselves are highlighted in green, indicating they are covered, but the actual properties within the data classes are not. This suggests that the issue lies within how KSP 2.3.x handles these annotations during coverage calculation, rather than a lack of actual test execution. This kind of unexpected behavior can be quite frustrating for developers, as it complicates the process of accurately assessing the coverage of their code. Therefore, a clear understanding of the underlying causes is essential for addressing the issue and ensuring reliable code coverage metrics.

The Bug: A Closer Look

The core of the problem lies in a change introduced between KSP versions 2.1.21-2.0.1 and 2.3.2. This change affects how KSP calculates coverage for data classes that utilize kotlinx-serialization attributes. The issue is illustrated by comparing coverage results between KSP 2.0.1 and 2.3.2. In version 2.0.1, a data class with kotlinx-serialization annotations would typically achieve 100% coverage, reflecting that all its properties and the class itself were being tested. However, in version 2.3.2, the coverage drops to 50%, despite the test suite remaining the same. This discrepancy is significant and indicates a problem with how KSP 2.3.x interprets or processes these annotations during the coverage calculation process. It's not that the code isn't being tested; rather, the coverage tool is misreporting the results, leading to an inaccurate assessment of the code's test coverage. This highlights the importance of carefully monitoring code coverage metrics after any updates to your toolchain and promptly investigating any unexpected changes.

In the provided example, the screenshots vividly demonstrate this issue. In both cases, only the annotation itself is highlighted in green, indicating coverage, while the actual properties within the data class are not. This suggests that KSP 2.3.x might be recognizing the presence of the annotation but not correctly tracking the execution of the associated data class members. This behavior is especially problematic because it creates a false sense of security. Developers might assume their code is adequately tested based on the 50% coverage figure, when in reality, a significant portion of the class's logic might not be covered. This discrepancy underscores the need for a fix to ensure that coverage reporting accurately reflects the state of testing within the application.

Impact and Affected Environments

This bug primarily affects projects using Kotlin, kotlinx-serialization, and KSP for code generation. It's particularly relevant in Kotlin Multiplatform projects, where code is shared across different platforms (e.g., Android and iOS). The inaccurate coverage reporting can lead to a false sense of security, potentially masking uncovered code paths and increasing the risk of bugs in production. The impact of this bug is most felt in projects that heavily rely on code coverage as a metric for ensuring code quality. When the coverage reports become unreliable, the entire testing strategy is undermined. Teams might make decisions based on incorrect data, leading to inadequate testing of critical components. For instance, developers might assume that a data class is fully tested based on the 50% coverage report, when in fact, crucial aspects of its functionality might be untested. This can result in overlooked bugs and a higher likelihood of issues making their way into production. Therefore, the consequences of this bug extend beyond mere reporting inaccuracies; they can directly impact the reliability and stability of the software.

Affected Environments and Configurations

Specifically, the bug has been observed in environments with the following configurations:

• Kover Gradle Plugin version: 0.8.3
• Gradle version: 9.1.0
• Kotlin project type: Kotlin/Multiplatform + Android
• Coverage Toolset: Kover

This combination of technologies is common in modern Kotlin development, making the bug relevant to a wide range of projects. The consistent recurrence of the issue under these conditions points to a specific interaction between KSP 2.3.x, kotlinx-serialization, and the coverage tooling. It's important to note that the issue is not limited to a specific type of project or application; rather, it's tied to the underlying technologies and versions being used. Therefore, any project that fits this configuration should be considered potentially affected and should be carefully evaluated for the presence of this coverage reporting problem. This underscores the importance of thoroughly testing and validating your code coverage metrics whenever you upgrade your dependencies or development tools.

Identifying the Issue

Identifying the coverage drop is relatively straightforward. After upgrading to KSP 2.3.x (or later) and running your coverage reports, you'll likely notice a significant decrease in coverage for data classes using kotlinx-serialization. As illustrated in the initial bug report, coverage might drop from 100% to 50% without any changes to the test suite. One of the most telltale signs of this issue is the discrepancy between the highlighted annotations and the actual properties within the data class. If the annotations are marked as covered (e.g., highlighted in green), but the properties themselves are not, this is a strong indication that you are encountering the bug. This visual cue is particularly helpful in quickly identifying the problem within your coverage reports. The inconsistency suggests that the coverage tool is recognizing the presence of the annotations but not accurately tracking the execution of the code within the data class itself. Therefore, paying close attention to these visual cues can help you promptly diagnose this issue and take appropriate action.

Steps to Confirm the Bug

To confirm whether you are affected, follow these steps:

1. Check KSP Version: Ensure you are using KSP 2.3.2 or a later version.
2. Review Coverage Reports: Examine your coverage reports, specifically focusing on data classes with kotlinx-serialization annotations.
3. Look for Discrepancies: Verify if the annotations are highlighted as covered while the class properties are not.
4. Compare with Previous Versions: If possible, compare coverage reports with a previous KSP version (e.g., 2.0.1) to see the difference.

By systematically following these steps, you can accurately determine whether your project is affected by this coverage issue. Starting with checking the KSP version ensures that you are looking for the bug in the relevant versions where it is known to occur. Reviewing the coverage reports and focusing on data classes with kotlinx-serialization narrows down the scope of your investigation to the most likely areas of impact. The crucial step is to look for discrepancies between annotation coverage and property coverage, as this is the hallmark of the bug. Finally, comparing reports with previous versions provides a clear baseline to confirm the drop in coverage and further validate the presence of the issue.

Potential Solutions and Workarounds

Currently, there isn't a definitive fix for this bug in KSP 2.3.x. However, several workarounds can help mitigate the issue and ensure accurate coverage reporting.

1. Downgrade KSP Version

The most straightforward workaround is to downgrade to a KSP version prior to 2.3.2, such as 2.0.1 or 2.1.21. This will revert the coverage calculation behavior to the previous, correct state. However, downgrading might mean missing out on other improvements and bug fixes in newer KSP versions. Downgrading KSP versions should be considered as a temporary solution, as it prevents you from leveraging the latest features and performance enhancements in the more recent releases. This approach is best suited for situations where accurate code coverage reporting is paramount, and the immediate benefits of the newer KSP versions are not critical. Before downgrading, it's important to assess the potential impact on other parts of your project and ensure compatibility with other dependencies. While it resolves the coverage issue, it's essential to keep an eye on KSP updates and plan to upgrade again once a fix is available.

2. Adjust Test Strategies

Another approach is to adjust your test strategies to explicitly target the properties of the data classes. This might involve writing more granular tests that specifically assert the behavior of each property. While this workaround doesn't fix the underlying bug, it ensures that your tests are actually covering the code, even if the coverage reports are inaccurate. Adjusting test strategies can be a beneficial long-term approach, as it often leads to more robust and comprehensive testing practices. By focusing on individual properties, you can ensure that each aspect of your data classes is thoroughly tested. This approach not only mitigates the immediate issue of inaccurate coverage reporting but also enhances the overall quality of your tests. However, it's important to recognize that this workaround requires a more manual effort in test design and implementation, and it may not fully compensate for the inaccurate coverage metrics provided by KSP 2.3.x.

3. Explore Alternative Coverage Tools

Consider exploring alternative code coverage tools that might handle kotlinx-serialization and KSP 2.3.x correctly. While Kover is a popular choice, other tools might provide more accurate results in this specific scenario. Exploring alternative coverage tools can offer a fresh perspective on the accuracy of your code coverage metrics. While Kover is widely used and respected, different tools may employ varying algorithms and techniques for calculating coverage, which could lead to different results. Before switching tools, it's essential to thoroughly evaluate the alternatives, considering factors such as ease of integration, compatibility with your project setup, and the level of detail provided in the coverage reports. It's also worth noting that even if an alternative tool provides more accurate coverage for this specific issue, it's still crucial to monitor KSP updates and consider switching back to your preferred tool once the bug is officially resolved.

4. Report the Issue and Monitor for Updates

Finally, if you haven't already, report the bug to the KSP development team. This helps ensure that the issue is addressed in a future release. Additionally, monitor the KSP issue tracker for updates and potential fixes. Reporting the issue is a crucial step in the bug resolution process. By providing detailed information about your environment, the steps to reproduce the bug, and the impact it's having on your project, you help the KSP team prioritize and address the issue effectively. Monitoring the issue tracker ensures that you stay informed about any progress made towards a fix and any potential workarounds or solutions that may emerge. This proactive approach helps you plan your updates and maintain a reliable development workflow.

Conclusion

The coverage drop issue in KSP 2.3.x when using kotlinx-serialization with data classes is a significant concern for Kotlin developers. By understanding the bug, its impact, and potential workarounds, you can take steps to mitigate the issue and ensure accurate code coverage reporting. Remember to report the bug and monitor for updates to ensure a smooth development experience. For more information on code coverage and testing strategies, check out resources like the Kotlin documentation on testing.