action.yml

name: Cascading Merge
description: Creates or updates a pull request to merge the current branch into the next one in alphabetic order.

inputs:
  branch_prefix:
    description: >
      Prefix of branches to consider when triggering the workflow or looking for the next branch to cascade to.
    default: 'maintenance/mps'
    required: false

  exclude_branch_prefix:
    description: Prefix of branches to exclude from consideration.
    required: false

  merge_prefix:
    description: >
      Prefix to use for merge branches created by the workflow. `branch_prefix` will be stripped before prepending
      the given prefix.
    default: 'merge/mps'
    required: false

  default_merge_prefix:
    description: Prefix to use for merge branch for merging to the default branch.
    default: 'merge/'
    required: false

  update_from_base:
    description: >
      Whether to update the merge branch from the base branch, as may be required by branch protection rules.
    default: 'false'
    required: false

  token:
    description: >
      GitHub token to use for repository operations. If not specified, the repository token will be used and operations
      performed by the action will not trigger any workflows. In particular, pull request checks will not run for PRs
      created or updated by the action.
    required: false

outputs:
  pr_number:
    description: "Number of the pull request for the merge created or updated by the action"
    value: ${{ steps.create-pr.outputs.pr_number }}

  pr_url:
    description: "URL of the pull request for the merge created or updated by the action"
    value: ${{ steps.create-pr.outputs.pr_url }}


runs:
  using: composite
  steps:
    - id: create-pr
      name: Create PR for new changes in ${{ github.ref_name }}
      shell: bash
      env:
        GH_TOKEN: ${{ inputs.token || github.token }}
        owner_and_repo: ${{ github.repository }}
        branch_prefix: ${{ inputs.branch_prefix }}
        default_merge_prefix: ${{ inputs.default_merge_prefix }}
        merge_prefix: ${{ inputs.merge_prefix }}
        update_from_base: ${{ inputs.update_from_base }}
        default_branch: ${{ github.event.repository.default_branch }}
      run: |
        jq_expr='.[] | .ref | ltrimstr("refs/heads/") | select(. > "'$GITHUB_REF_NAME'")'
        if [[ -n '${{ inputs.exclude_branch_prefix }}' ]]
        then
          jq_expr+=' | select(startswith("${{ inputs.exclude_branch_prefix }}") | not)'
        fi
        jq_expr='['$jq_expr'] | min'

        echo "Calculating PR branches:"

        # Next branch matching refs/heads/${{ inputs.branch_prefix}}, or empty if we're on the last one
        next_maintenance_branch=$(gh api "/repos/$owner_and_repo/git/matching-refs/heads/${{ inputs.branch_prefix }}" \
          --jq "$jq_expr")
        echo "  Next maintenance branch: ${next_maintenance_branch:-(none)}"

        # Substitute the default branch if empty
        base_branch=${next_maintenance_branch:-$default_branch}
        echo "  PR base branch: $base_branch"

        # Calculate PR head branch name (the branch to merge) by substituting 'maintenance' with 'merge'
        head_branch="$GITHUB_REF_NAME"
        
        if [[ "$base_branch" == "$default_branch" ]]
        then
          merge_branch="$default_merge_prefix$default_branch"
        else
          merge_branch=${base_branch/#$branch_prefix/$merge_prefix}
        fi
        echo "  Merge branch (will be used as PR head): $merge_branch"

        echo ""

        echo "Pushing $GITHUB_REF_NAME to $merge_branch"

        # Create or update $merge_branch
        if gh api repos/$owner_and_repo/git/ref/heads/$merge_branch --silent
        then
          # Branch exists, merge $head_branch into it:

          # First, try fast-forwarding
          echo "Updating $merge_branch to SHA $GITHUB_SHA"
          gh api repos/$owner_and_repo/git/refs/heads/$merge_branch --silent --method=PATCH -f sha="$GITHUB_SHA" \
            -F force=false ||

            # If that failed, try merging
            (echo "Update failed, trying to merge $head_branch into $merge_branch" ;
              gh api repos/$owner_and_repo/merges --silent --method=POST -f base=$merge_branch -f head=$head_branch) ||

              # If that failed, report an error
              (echo "Merging $head_branch to existing $merge_branch failed, aborting." ; exit 1)
        else
          # Branch does not exist, create it from $head_branch
          gh api repos/$owner_and_repo/git/refs --silent --method=POST -f ref="refs/heads/$merge_branch" -f sha="$GITHUB_SHA" ||
            (echo "Could not create merge branch $merge_branch" ; exit 1)
        fi

        # Update $base_branch from $base_branch, if requested
        if [[ "$update_from_base" == true ]]
        then
          # The head and base arguments are intentionally reversed because this is a reverse merge.
          gh api repos/$owner_and_repo/merges --silent --method=POST -f base=$merge_branch -f head=$base_branch ||
            echo "Updating $merge_branch from $base_branch failed, please do it manually."
        fi

        # Create PR if it does not exist yet
        read -r pr_number pr_url < <(gh pr list --repo $owner_and_repo --head $merge_branch --base $base_branch \
          --json number,url --jq 'map(.number, .url) | join(" ")')

        if [[ $pr_number ]]
        then
          echo "Found existing PR #$pr_number for $merge_branch -> $base_branch ($pr_url)"

          echo "Requesting review on PR #$pr_number from $GITHUB_ACTOR"
          gh api repos/$owner_and_repo/pulls/$pr_number/requested_reviewers \
            --silent --method=POST -f "reviewers[]=$GITHUB_ACTOR" || true

          echo "Assigning PR #$pr_number to $GITHUB_ACTOR"
          gh api repos/$owner_and_repo/issues/$pr_number/assignees \
            --silent --method=POST -f "assignees[]=$GITHUB_ACTOR" || true

        else
          echo "Creating a new pull request $merge_branch -> $base_branch"
          gh pr create --repo $owner_and_repo --head $merge_branch --base $base_branch \
            --title "Merge $GITHUB_REF_NAME into $base_branch" \
            --body "Merge changes from \`$GITHUB_REF_NAME\` to \`$base_branch\`." \
            --assignee "$GITHUB_ACTOR" \
            --reviewer "$GITHUB_ACTOR"

          read -r pr_number pr_url < <(gh pr list --repo $owner_and_repo --head $merge_branch --base $base_branch \
            --json number,url --jq 'map(.number, .url) | join(" ")')
        fi

        echo "pr_number=$pr_number" >> $GITHUB_OUTPUT
        echo "pr_url=$pr_url" >> $GITHUB_OUTPUT