Investigating Code Review Practices in Defective Files: An Empirical Study of the Qt System

Software code review is a well-established software
quality practice. Recently, Modern Code Review (MCR) has been
widely adopted in both open source and proprietary projects.
To evaluate the impact that characteristics of MCR practices
have on software quality, this paper comparatively studies MCR
practices in defective and clean source code files. We investigate
defective files along two perspectives: 1) files that will eventually
have defects (i.e., future-defective files) and 2) files that have
historically been defective (i.e., risky files). Through an empirical
study of 11,736 reviews of changes to 24,486 files from the Qt
open source project, we find that both future-defective files and
risky files tend to be reviewed less rigorously than their clean
counterparts. We also find that the concerns addressed during
the code reviews of both defective and clean files tend to enhance
evolvability, i.e., ease future maintenance (like documentation),
rather than focus on functional issues (like incorrect program
logic). Our findings suggest that although functionality concerns
are rarely addressed during code review, the rigor of the
reviewing process that is applied to a source code file throughout
a development cycle shares a link with its defect proneness.