Github Action to keep fork updated with upstream

sync-fork.yaml

# Script is (loosely) based on two references:
# * https://stackoverflow.com/questions/23793062/can-forks-be-synced-automatically-in-github
# * https://stackoverflow.com/questions/69918635/how-to-keep-all-branches-and-tags-in-sync-in-a-fork-or-mirror-repo
# [!] Note: Do not do a force push to not overwrite the .github/workflow/main.yml file.

# We are not using a predefined action (eg. [1]) to not go through the hassle of having to
# manage Github Personal Access Tokens for deephdc.
# [1]: https://github.com/repo-sync/github-sync


name: Sync fork with upstream
on:
  schedule:
     # run at max frequency
    - cron:  '* * * * *'

  # Allows you to run this workflow manually from the Actions tab
  workflow_dispatch:

jobs:

  build:
    
    runs-on: ubuntu-latest

    steps:
      # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
      - uses: actions/checkout@v3
        with:
          fetch-depth: 0

      - name: Sync fork with upstream
        run: |
          
          #########################################################
          # YOU SHOULD UPDATE THIS LINE
          #########################################################

          UPSTREAM_REPO=https://github.com/<username>/<reponame>.git

          #########################################################

          # Bot config
          git config user.name 'github-actions[bot]'
          git config user.email 'github-actions[bot]@users.noreply.github.com'
                    
          # Add remote
          git remote add upstream $UPSTREAM_REPO
          git fetch upstream

          # Keep track of branch names
          origin_branches=$(git branch -r | grep -v 'HEAD' | grep 'origin/' | cut -f 2 -d '/')
          upstream_branches=$(git branch -r | grep 'upstream/' | cut -f 2 -d '/')
          
          old_branches=$(comm -13 <(printf '%s\n' "${upstream_branches[@]}" | LC_ALL=C sort) <(printf '%s\n' "${origin_branches[@]}" | LC_ALL=C sort))
          new_branches=$(comm -13 <(printf '%s\n' "${origin_branches[@]}" | LC_ALL=C sort) <(printf '%s\n' "${upstream_branches[@]}" | LC_ALL=C sort))
          existing_branches=$(comm -13 <(printf '%s\n' "${new_branches[@]}" | LC_ALL=C sort) <(printf '%s\n' "${upstream_branches[@]}" | LC_ALL=C sort))
          
           # Delete old branches from origin
          echo "# Deleting old branches ..."
          for tmp_branch in $old_branches; do
            echo "## Processing $tmp_branch ..."
            git push origin --delete $tmp_branch
          done
          
          # Create origin branches for new upstream branches
          echo "# Creating new branches ..."
          for tmp_branch in $new_branches; do
            echo "## Processing $tmp_branch ..."
            git checkout -b $tmp_branch upstream/$tmp_branch
            git push origin
          done
          
          # Merge changes from upstream to origin for existing branches
          echo "# Merging existing branches ..."
          git config --add checkout.defaultRemote origin
          for tmp_branch in $existing_branches; do
            echo "## Processing $tmp_branch ..."
            git checkout $tmp_branch
            git merge --no-edit upstream/$tmp_branch
            git push origin
          done
          
          # Sync tags
          git tag -d $(git tag -l)
          git fetch upstream --tags --quiet
          git push origin --tags --force