How do Multiple Pull Requests Change the Same Code: A Study of Competing Pull Requests in GitHub

GitHub is a widely used collaborative platform for global software development. A pull request plays an important role in bridging code changes with version controlling. Developers can freely and parallelly submit pull requests to base branches and wait for the merge of their contributions. However, several developers may submit pull requests to edit the same lines of code; such pull requests result in a latent collaborative conflict. We refer such pull requests that tend to change the same lines and remain open during an overlapping time period to as competing pull requests. In this paper, we conduct a study on 9,476 competing pull requests from 60 Java repositories in GitHub. The data are collected by mining pull requests that are submitted in 2017 from top Java projects with the most forks. We explore how multiple pull requests change the same code via answering four research questions, including the distribution of competing pull requests, the involved developers, the changed lines of code, and the impact on pull request integration. Our study shows that there indeed exist competing pull requests in GitHub: in 45 out of 60 repositories, over 31% of pull requests belong to competing pull requests; 20 repositories have more than 100 groups of competing pull requests, each of which is submitted by over five developers; 42 repositories have over 10% of competing pull requests with over 10 same lines of code. Meanwhile, we observe that attributes of competing pull requests do not have strong impacts on pull request integration, comparing with other types of pull requests. Our study provides a preliminary analysis for further research that aims to detect and eliminate conflicts among competing pull requests.