How To: Clone target instead of source branch for pull request

This article describes how to clone the target instead of the source branch in a PR (pull request)-triggered build.

Typically, pipeline builds use the CF_BRANCH variable to reference the branch associated with a build triggered by a Git event. However, in the case of a build triggered by a PR, CF_BRANCH specifically references the source branch. To clone the target branch instead of the source branch during a merge, use the CF_PULL_REQUEST_TARGET variable, which specifically points to the target branch.

How to

NOTE
CF_BRANCH is available only for PR-triggered builds. Updating its value to match CF_PULL_REQUEST_TARGET clones the target branch instead.
If CF_PULL_REQUEST_TARGET is not present, the build defaults to cloning the source branch.

• Create a separate pipeline that uses ${{CF_PULL_REQUEST_TARGET}} instead of the standard ${{CF_BRANCH}}
  OR

• Add the following step before your git-clone step to check if CF_PULL_REQUEST_TARGET exists.
  If it does, to assign its value to CF_BRANCH:

    update_branch_var:
      type: freestyle
      description: "Updating clone branch var to PR target branch"
      stage: 'clone'
      arguments:
        image: 'alpine:3.8'
        commands:
          - cf_export CF_BRANCH='${{CF_PULL_REQUEST_TARGET}}'
        when:
          condition:
            all:
              whenTargetExists: 'includes("{{CF_PULL_REQUEST_TARGET}}", "{{CF_PULL_REQUEST_TARGET}}") == false'