name: fast-forward for PR
# GitHub doesn't support fast-forward merge of PR, 
# which causes GPG signatures overriden by GitHub's key.
# This workflow allows maintainers to do a fast-forward merge
# by replying anything containing `/fast-forward` to the PR.
#
# Contributors must first rebase onto the latest commit before PR to make fast-forward possible.
on:
  # issue / PR comment
  issue_comment:
    types: [created, edited]
  # PR comment in review
  pull_request_review_comment:
    types: [created, edited]
jobs:
  fast-forward:
    if: ${{ contains(github.event.comment.body, '/fast-forward')
            && github.event.issue.pull_request }}
    runs-on: ubuntu-latest
    permissions:
      contents: write
      pull-requests: write

    steps:
      - name: Check User Permission
        id: check-perm
        uses: actions-cool/check-user-permission@v2.2.0
        with:
          require: write

      - name: Fast forwarding
        if: steps.check-perm.outputs.require-result == 'true'
        # As GitHub actions use actor's permission, it's not possible
        # for anyone without write permission to perform a merge.
        # This check is merely for skipping it silently.
        uses: sequoia-pgp/fast-forward@v1
        with:
          merge: true
          comment: on-error