name: Check for missing commits

# TODO: Use `workflow_run` to trigger this workflow after checks have completed.
# This can only be done if the `checks` workflow already exists on the default
# branch (i.e. `develop`), so we cannot do this yet.
# See https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#workflow_run.
on:
  push:
    branches:
      - develop
      - release

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

defaults:
  run:
    shell: bash

jobs:
  check:
    runs-on: ubuntu-latest
    steps:
    - name: Checkout repository
      uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
      with:
        fetch-depth: 0
    - name: Check for missing commits
      env:
        MESSAGE: |

          If you are reading this, then the commits indicated above are missing
          from the "develop" and/or "release" branch. Do a reverse-merge as soon
          as possible. See CONTRIBUTING.md for instructions.
      run: |
        set -o pipefail
        # Branches are ordered by how "canonical" they are. Every commit in one
        # branch should be in all the branches behind it.
        order=(master release develop)
        branches=()
        for branch in "${order[@]}"; do
          # Check that the branches exist so that this job will work on forked
          # repos, which don't necessarily have master and release branches.
          echo "Checking if ${branch} exists."
          if git ls-remote --exit-code --heads origin \
            refs/heads/${branch} > /dev/null; then
            branches+=(origin/${branch})
          fi
        done

        prior=()
        for branch in "${branches[@]}"; do
          if [[ ${#prior[@]} -ne 0 ]]; then
            echo "Checking ${prior[@]} for commits missing from ${branch}."
            git log --oneline --no-merges "${prior[@]}" \
              ^$branch | tee -a "missing-commits.txt"
            echo
          fi
          prior+=("${branch}")
        done
        
        if [[ $(cat missing-commits.txt | wc -l) -ne 0 ]]; then
          echo "${MESSAGE}"
          exit 1
        fi