Problems using github-branch-source plugin for pull request jobs

[arw62]

We discovered some significant issues with using the GitHub Branch Source Plugin for Pull request jobs in ros_buildfarm. The goal was to first attempt to manually reconfigure a pull request job to use the github-branch-source plugin instead of the legacy GitHub Pull Request Builder (Ghprb) plugin and then use the modified job as a baseline for updating the pull request jobs in ros_buildfarm.

Our first attempt to prototype using the github-branch-source plugin in a Pull request job was unsuccessful because the Branch Sources configuration block is not present in Freestyle jobs, the generic Jenkins job type that all of our jobs use.

Our next attempt to use the github-branch-source plugin had us following the Jenkins book section on Branches and pull requests to try and set up a proof-of-concept PR job based on the multibranch pipeline job type in that article. We reviewed and verified the presence of the pull request-related Behaviors which would allow us to run jobs based on pull requests. However the Build Configuration section in multibranch pipeline jobs support only job configuration by Jenkinsfile which prevents us from using centralized build configurations.

In order to migrate to the github-branch-source plugin for pull request jobs either Freestyle jobs need to support the Branch Sources configuration section which allows for the pull request testing behavior to be configured or the Multibranch pipeline job type needs to support a Freestyle-like Build Configuration section which would allow us to continue using our ros_buildfarm job templates.