What are the Characteristics of Reopened Pull Requests? A Case Study on Open Source Projects in GitHub

In GitHub, integrators inspect submitted code changes, make evaluation decision, and close pull requests. Some pull requests may be reopened for further modification and code review. It remains unknown why some pull requests are reopened, and how they affect software development. In this paper, we conduct a case study to understand reopened pull requests. We collect 100,622 pull requests from 7 popular projects in GitHub. We study the impacts of reopened pull requests on the code review process. We then qualitatively analyze the root reasons that lead to reopening pull requests. Finally, we examine the characteristics of pull requests caused by various reasons. Our main observations are: 1) Reopened pull requests have lower acceptance rates, more comments, and longer evaluation time than non-reopened pull requests. 2) Several key reasons that lead to reopened pull request are identified such as
bugs
,
change minds
,
insufficient tests
,
incompatible version
,
tests fails
, and so on. 3) Reopened pull requests in different categories have various acceptance rates, comment amounts, and evaluation time. Reopened pull requests in the category
rebase and change branch
has the largest number of comments and the longest evaluation time. These findings provide insights to decrease reopened pull requests, develop a variety of assistant tools, and reduce the maintenance cost of open source projects.