name: Close PR From Fork Default Branch

on:
  # pull_request_target is required so we have permission to comment and close PRs from forks.
  pull_request_target:
    types: [opened, reopened]

permissions:
  pull-requests: write
  issues: write

jobs:
  close:
    name: Close PR opened from fork's default branch
    runs-on: ubuntu-latest
    if: >-
      github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
      && github.event.pull_request.head.ref == github.event.repository.default_branch
    steps:
      - uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
        with:
          script: |
            const { owner, repo } = context.repo;
            const prNumber = context.payload.pull_request.number;
            const author = context.payload.pull_request.user.login;
            const defaultBranch = context.payload.repository.default_branch;
            const headRepo = context.payload.pull_request.head.repo.full_name;

            const body = [
              `Hi @${author}, thanks for opening a pull request! :tada:`,
              ``,
              `It looks like this PR was opened from the \`${defaultBranch}\` branch of your fork (\`${headRepo}\`), which is the same name as this repository's default branch. Working directly on \`${defaultBranch}\` in your fork causes a few problems:`,
              ``,
              `- Your fork's \`${defaultBranch}\` branch will permanently diverge from \`${owner}/${repo}:${defaultBranch}\`, making it hard to keep your fork up to date.`,
              `- Any additional commits you push to \`${defaultBranch}\` will be added to this PR, so you can't easily work on multiple changes at once.`,
              `- Pushing maintainer fixes to your branch is awkward, since it means committing directly to your fork's default branch.`,
              `- It makes local collaboration painful — \`${defaultBranch}\` in a checkout becomes ambiguous between upstream and your fork, and maintainers end up with naming collisions when fetching your branch.`,
              ``,
              `Please re-open this as a new PR from a dedicated feature branch. The usual flow looks like:`,
              ``,
              `\`\`\`bash`,
              `# Make sure your fork's ${defaultBranch} is up to date with upstream`,
              `git remote add upstream https://github.com/${owner}/${repo}.git  # if you haven't already`,
              `git fetch upstream`,
              `git checkout ${defaultBranch}`,
              `git reset --hard upstream/${defaultBranch}`,
              `git push --force-with-lease origin ${defaultBranch}`,
              ``,
              `# Create a new branch for your change and cherry-pick / re-apply your commits there`,
              `git checkout -b my-feature-branch upstream/${defaultBranch}`,
              `# ...re-apply your changes, then:`,
              `git push origin my-feature-branch`,
              `\`\`\``,
              ``,
              `Then open a new pull request from \`my-feature-branch\` into \`${owner}/${repo}:${defaultBranch}\`.`,
              ``,
              `Closing this PR for now — sorry for the friction, and thanks again for contributing! :heart:`,
            ].join('\n');

            await github.rest.issues.createComment({
              owner,
              repo,
              issue_number: prNumber,
              body,
            });

            await github.rest.pulls.update({
              owner,
              repo,
              pull_number: prNumber,
              state: 'closed',
            });