til/github-actions/use-labels-to-block-pr-merge.md

Use Labels To Block PR Merge

Let's say our GitHub project has custom tags for both no merge and wip (work in progress). Whenever either of those labels has been applied to a PR, we want there to be a failed check so as to block the merge. This is useful to ensure automated tools (as well as someone not looking closely enough) don't merge a PR that isn't ready to go.

This can be achieved with a basic GitHub Actions workflow that requires no 3rd-party actions. We can add the following as .github/workflows/block-labeled-prs.yml in our project.

name: Block Labeled PR Merges

on:
  pull_request:
    types: [labeled, unlabeled, opened, edited, synchronize]

jobs:
  prevent-merge:
    if: ${{ contains(github.event.*.labels.*.name, 'no merge') || contains(github.event.*.labels.*.name, 'wip') }}
    name: Prevent Merging
    runs-on: ubuntu-latest
    steps:
      - name: Check for label
        run: |
          echo "Pull request label prevents merging."
          echo "Labels: ${{ join(github.event.*.labels.*.name, ', ') }}"
          echo "Remove the blocking label(s) to skip this check."
          exit 1

This workflow is run when a pull request is opened, when it is edited or synchronized, and when a label change is made. The job prevent-merge sees if any of the label names match no merge or wip. If so, we echo out some details in the ubuntu container and then exit 1 to fail the check.

Shoutout to Jesse Squire's implementation which I've heavily borrowed from here.