name: Rebase Pull Request

# Ticking the "Rebase" checkbox of the pull request description rebases the
# branch on its base branch, only if it applies without conflict.
#
# `pull_request_target` gives a token able to write on pull requests coming
# from forks. It is safe here because the pull request code is never checked
# out nor run, and the description is only read through the environment.
on:
  pull_request_target:
    types: [edited]

permissions:
  pull-requests: write

# One run per pull request, never cancelled: a rebase must not be interrupted.
concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
  cancel-in-progress: false

jobs:
  rebase:
    runs-on: ubuntu-latest
    if: contains(github.event.pull_request.body, '[x] Rebase this pull request')
    env:
      GH_TOKEN: ${{ github.token }}
      GH_REPO: ${{ github.repository }}
      PR_NUMBER: ${{ github.event.pull_request.number }}
      HEAD_SHA: ${{ github.event.pull_request.head.sha }}
    steps:
      - name: Untick the checkbox
        # First, so that the edit of the description cannot run this again.
        run: |
          gh api "repos/$GH_REPO/pulls/$PR_NUMBER" --jq .body > body.md
          sed -i -E 's/\[[xX]\] (Rebase this pull request)/[ ] \1/' body.md
          gh api -X PATCH "repos/$GH_REPO/pulls/$PR_NUMBER" -F body=@body.md > /dev/null

      - name: Rebase on the base branch
        run: |
          # Only rebases if HEAD_SHA is still the head of the branch.
          if gh api -X PUT "repos/$GH_REPO/pulls/$PR_NUMBER/update-branch" \
            -f update_method=rebase -f expected_head_sha="$HEAD_SHA" \
            > result.txt 2>&1; then
            message=":white_check_mark: The branch has been rebased on its base branch. Its commits were rewritten, so \`git pull --rebase\` before pushing again, and note that the signatures of the rebased commits are lost."
          elif grep -qi "conflict" result.txt; then
            message=":x: The rebase is not possible because of conflicts with the base branch. It has to be done manually."
          elif grep -qi "no new commits" result.txt; then
            message=":information_source: The branch is already up to date with its base branch."
          else
            message=":warning: The automatic rebase failed (for a pull request from a fork, the option \"Allow edits by maintainers\" must be enabled). It has to be done manually."
            cat result.txt
          fi
          gh pr comment "$PR_NUMBER" --body "$message"