action.yml

name: Sync branches
description: On a labeled pull request, create a companion sync branch and open a pull request that proposes merging the source branch into a target environment branch.
author: Human Made

inputs:
  from_branch:
    description: Source branch to sync from — the head branch of the labeled pull request.
    required: true
  to_branch:
    description: Target branch the new sync pull request opens against (e.g. dev, staging).
    required: true
  required_label:
    description: Label on the source pull request that triggered the sync. Removed from the source PR before creating the sync PR.
    required: true
  new_branch_suffix:
    description: Suffix appended to from_branch to form the new sync branch name.
    default: dev
    required: false
  pull_request_title:
    description: Title of the sync pull request. Defaults to "sync: {from_branch} to {to_branch}".
    default: ""
    required: false
  pull_request_body:
    description: Body of the sync pull request. Defaults to "sync-branches: Merge #{source_pr} to {to_branch}".
    default: ""
    required: false
  pull_request_is_draft:
    description: Create the sync pull request as a draft.
    default: "false"
    required: false
  github_token:
    description: Token used for GitHub API calls. Defaults to the job's GITHUB_TOKEN.
    default: ${{ github.token }}
    required: false

outputs:
  pull_request_url:
    description: HTML URL of the sync pull request (either newly created or pre-existing).
    value: ${{ steps.sync.outputs.pull_request_url }}
  pull_request_number:
    description: Number of the sync pull request (either newly created or pre-existing).
    value: ${{ steps.sync.outputs.pull_request_number }}

runs:
  using: composite

  steps:
    - name: Create sync branch and pull request
      id: sync
      shell: bash
      env:
        GH_TOKEN: ${{ inputs.github_token }}
      run: |
        set -euo pipefail

        FROM_BRANCH="${{ inputs.from_branch }}"
        TO_BRANCH="${{ inputs.to_branch }}"
        REQUIRED_LABEL="${{ inputs.required_label }}"
        NEW_BRANCH_SUFFIX="${{ inputs.new_branch_suffix }}"
        PR_TITLE_INPUT="${{ inputs.pull_request_title }}"
        PR_BODY_INPUT="${{ inputs.pull_request_body }}"
        PR_IS_DRAFT=$(echo "${{ inputs.pull_request_is_draft }}" | tr '[:upper:]' '[:lower:]')

        REPO="${{ github.repository }}"
        ORIG_PR_NUMBER="${{ github.event.pull_request.number }}"
        ORIG_PR_AUTHOR="${{ github.event.pull_request.user.login }}"

        if [ -z "$ORIG_PR_NUMBER" ]; then
          echo "::error::This action must be called from a pull_request event; github.event.pull_request.number is empty."
          exit 1
        fi

        NEW_BRANCH="${FROM_BRANCH}-${NEW_BRANCH_SUFFIX}"

        # Remove the triggering label so the workflow can't re-fire on the source PR.
        # If the label is already gone (e.g. manually removed), skip the sync quietly.
        if ! gh pr edit "$ORIG_PR_NUMBER" --repo "$REPO" --remove-label "$REQUIRED_LABEL"; then
          echo "Could not remove label '$REQUIRED_LABEL' from PR #$ORIG_PR_NUMBER; skipping sync."
          exit 0
        fi

        # Fail fast if the sync branch already exists — otherwise we'd silently
        # diverge from a stale branch.
        if gh api "repos/$REPO/branches/$NEW_BRANCH" --silent 2>/dev/null; then
          gh pr comment "$ORIG_PR_NUMBER" --repo "$REPO" \
            --body "Sync has failed as branch \`$NEW_BRANCH\` already exists. Please delete the branch and restart the workflow."
          echo "::error::Sync branch '$NEW_BRANCH' already exists; delete it and re-run."
          exit 1
        fi

        # Create the sync branch at the source PR's head commit.
        HEAD_SHA=$(gh pr view "$ORIG_PR_NUMBER" --repo "$REPO" --json headRefOid -q .headRefOid)
        gh api -X POST "repos/$REPO/git/refs" \
          -f "ref=refs/heads/$NEW_BRANCH" \
          -f "sha=$HEAD_SHA" > /dev/null

        # If a sync PR is already open from this exact branch pair, return it.
        EXISTING=$(gh pr list --repo "$REPO" \
          --head "$NEW_BRANCH" --base "$TO_BRANCH" \
          --state open --json number,url --limit 1)
        EXISTING_NUMBER=$(echo "$EXISTING" | jq -r '.[0].number // empty')
        EXISTING_URL=$(echo "$EXISTING" | jq -r '.[0].url // empty')
        if [ -n "$EXISTING_NUMBER" ]; then
          echo "Sync PR already exists: #$EXISTING_NUMBER"
          echo "pull_request_number=$EXISTING_NUMBER" >> "$GITHUB_OUTPUT"
          echo "pull_request_url=$EXISTING_URL" >> "$GITHUB_OUTPUT"
          exit 0
        fi

        PR_TITLE="${PR_TITLE_INPUT:-sync: ${FROM_BRANCH} to ${TO_BRANCH}}"
        PR_BODY="${PR_BODY_INPUT:-sync-branches: Merge #${ORIG_PR_NUMBER} to ${TO_BRANCH}}"

        NEW_PR=$(gh api -X POST "repos/$REPO/pulls" \
          -f "title=$PR_TITLE" \
          -f "body=$PR_BODY" \
          -f "head=$NEW_BRANCH" \
          -f "base=$TO_BRANCH" \
          -F "draft=$PR_IS_DRAFT")
        NEW_PR_NUMBER=$(echo "$NEW_PR" | jq -r .number)
        NEW_PR_URL=$(echo "$NEW_PR" | jq -r .html_url)

        # Assign the sync PR to the original PR author so they can follow up.
        gh pr edit "$NEW_PR_NUMBER" --repo "$REPO" --add-assignee "$ORIG_PR_AUTHOR" || \
          echo "Warning: could not assign PR #$NEW_PR_NUMBER to $ORIG_PR_AUTHOR."

        echo "Created sync PR #$NEW_PR_NUMBER: $NEW_PR_URL"
        echo "pull_request_number=$NEW_PR_NUMBER" >> "$GITHUB_OUTPUT"
        echo "pull_request_url=$NEW_PR_URL" >> "$GITHUB_OUTPUT"