name: Cancel builds of closed pull requests

on:
  # pull_request_target needs no approval for first-time contributors and must never
  # check out pull request code.
  pull_request_target:
    types:
      - closed

permissions: {}

jobs:
  cancel:
    # Matches the concurrency group in build_all.yml, so joining it cancels the pull
    # request's unfinished Build all run.
    concurrency:
      group: build-all-pull_request-${{ github.event.pull_request.number }}
      cancel-in-progress: true
    runs-on: ubuntu-latest
    steps:
      - run: echo "Joined concurrency group build-all-pull_request-${{ github.event.pull_request.number }}."