Existing maintainability models are used to identify technical debt of software systems. Targeting entire codebases, such models lack the ability to determine shortcomings of smaller, fine-grained changes. This paper proposes a new maintainability model-the Delta Maintainability Model (DMM)-to measure fine-grained code changes, such as commits, by adapting and extending the SIG Maintainability Model. DMM categorizes changed lines of code into low and high risk, and then uses the proportion of low risk change to calculate a delta score. The goal of the DMM is twofold: first, producing meaningful and actionable scores; second, compare and rank the maintainability of fine-grained modifications. We report on an initial study of the model, with the goal of understanding if the adapted measurements from the SIG Maintainability Model suit the fine-grained scope of the DMM. In a manual inspection process for 100 commits, 67 cases matched the expert judgment. Furthermore, we report an exploratory empirical study on a data set of DMM scores on 3,017 issue-fixing commits of four open source and four closed source systems. Results show that the scores of DMM can be used to compare and rank commits, providing developers with a means to do root cause analysis on activities that impacted maintainability and, thus, address technical debt at a finer granularity.

Modern Code Review (MCR) is an established software development process that aims to improve software quality. Although evidence showed that higher levels of review coverage relates to less post-release bugs, it remains unknown the effectiveness of MCR at specifically finding security issues.We present a work we conduct aiming to fill that gap by exploring the MCR process in the Chromium open source project. We manually analyzed large sets of registered (114 cases) and missed (71 cases) security issues by backtracking in the project's issue, review, and code histories. This enabled us to qualify MCR in Chromium from the security perspective from several angles: Are security issues being discussed frequently? What categories of security issues are often missed or found? What characteristics of code reviews appear relevant to the discovery rate?Within the cases we analyzed, MCR in Chromium addresses security issues at a rate of 1% of reviewers' comments. Chromium code reviews mostly tend to miss language-specific issues (e.g., C++ issues and buffer overflows) and domain-specific ones (e.g., such as Cross-Site Scripting); when code reviews address issues, mostly they address those that pertain to the latter type. Initial evidence points to reviews conducted by more than 2 reviewers being more successful at finding security issues.

Background. Code review is a cognitively demanding and time-consuming process. Previous qualitative studies hinted at how decomposing change sets into multiple yet internally coherent ones would improve the reviewing process. So far, no quantitative analysis of this hypothesis has been provided. Aims. (1) Quantitatively measure the effects of change decomposition on the outcome of code review (in terms of number of found defects, wrongly reported issues, suggested improvements, time, and understanding); (2) Qualitatively analyze how subjects approach the review and navigate the code, building knowledge and addressing existing issues, in large vs. decomposed changes. Method. Controlled experiment using the pull-based development model involving 28 software developers among professionals and graduate students. Results. Change decomposition leads to fewer wrongly reported issues, influences how subjects approach and conduct the review activity (by increasing context-seeking), yet impacts neither understanding the change rationale nor the number of found defects. Conclusions. Change decomposition reduces the noise for subsequent data analyses but also significantly supports the tasks of the developers in charge of reviewing the changes. As such, commits belonging to different concepts should be separated, adopting this as a best practice in software engineering.

Background. Code review is a cognitively demanding and time-consuming process. Previous qualitative studies hinted at how decomposing change sets into multiple yet internally coherent ones would improve the reviewing process. So far, no quantitative analysis of this hypothesis has been provided. Aims. (1) Quantitatively measure the effects of change decomposition on the outcome of code review (in terms of number of found defects, wrongly reported issues, suggested improvements, time, and understanding); (2) Qualitatively analyze how subjects approach the review and navigate the code, building knowledge and addressing existing issues, in large vs. decomposed changes. Method. Controlled experiment using the pull-based development model involving 28 software developers among professionals and graduate students. Results. Change decomposition leads to fewer wrongly reported issues, influences how subjects approach and conduct the review activity (by increasing context-seeking), yet impacts neither understanding the change rationale nor the number of found defects. Conclusions. Change decomposition reduces the noise for subsequent data analyses but also significantly supports the tasks of the developers in charge of reviewing the changes. As such, commits belonging to different concepts should be separated, adopting this as a best practice in software engineering.