action.yml

name: Reverse Rebase from Upstream
description: Use git rebase to append new upstream commits
# inverted logic from imba-tjd/rebase-on-upstream
branding:
  icon: git-pull-request
  color: blue

inputs:
  upstream:
    type: string
    description: <user>/<repo> or full HTTP URL; default is parent repo
    required: false
  branch:
    type: string
    description: The upstream branch that is rebased from
    required: false
    default:  master
  push:
    type: boolean
    description: Do the force push in this action
    required: false
    default:  true
  token:
    type: string
    description: GitHub Access Token
    required: true
    default:  ${{ github.token }}

permissions:
  contents: write

runs:
  using: composite
  steps:
    - name: Main
      shell: bash
      run: |
        set -ex
        # Starting with current git branch (TARGET), get the UPSTREAM branch
        # as a temporary branch; rebase TARGET onto it; if that differs from TARGET,
        # make it the TARGET and push the result, if the remote hasn't changed.
        # Effect: commits specific to the TARGET repo stay in place; new UPSTREAM
        # commits are appended
        TARGET_SHA=$(git rev-parse HEAD)
        TARGET=$(git name-rev --name-only --exclude='tags/*' "$TARGET_SHA")
        if [ -z "$TARGET" ]; then echo "Can't find current branch" >&2; exit 1; fi
        UPSTREAM=${{ inputs.upstream }}
        if [ -z "$UPSTREAM" ]; then
          echo "${{ inputs.token }}" | gh auth login --with-token
          UPSTREAM=$(gh api repos/:owner/:repo --jq .parent.full_name)
          if [ -z "$UPSTREAM" ]; then echo "Can't find upstream" >&2 && exit 1; fi
        fi
        if ! echo "$UPSTREAM" | grep -E '^(http|git@)'; then
          UPSTREAM=https://github.com/$UPSTREAM.git
        fi
        git remote add -f -t "${{ inputs.branch }}" --tags upstream "$UPSTREAM"
        REBASE=${TARGET}-rebase
        # avoid leaving an unwanted branch "$REBASE"
        trap '{ git checkout "$TARGET"; git branch -D "$REBASE" || true; }' EXIT
        git checkout -B  "$REBASE" --no-track "upstream/${{ inputs.branch }}" 
        git config --local user.email "rebaser+github-actions[bot]@users.noreply.github.com"
        git config --local user.name  "GitHub Actions"
        # rebase new UPSTREAM commits onto TARGET
        git rebase "refs/heads/${TARGET}"
        if [ -n "$(git log --max-count 1 --oneline "HEAD...${TARGET}")"  ]; then
          git branch -M "$REBASE" "$TARGET"
          if [ "${{ inputs.push }}" = true ]; then
            # rebase changed TARGET: push if remote TARGET unchanged
            git push origin --force-with-lease="${TARGET}"
          fi
        fi