This Action is suitable if you:

• are maintaining a fork
• have changes that are not going to be merged into upstream
• want to keep changes based on the latest upstream

Basically this is doing git rebase upstream master && git push -f. If there are conflicts, it simply fails.

# .github/workflows/sync.yml
name: Rebase Upstream
on:
  schedule:
  - cron: "0 0 * * 0"  # run once a week
  workflow_dispach:    # run manually

jobs:
  sync:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@master
      with:
        fetch-depth: 10  # greater than the number of commits you made
    - uses: imba-tjd/rebase-upstream-action@master
      # with:  # all args are optional
      #   upstream: <user>/<repo>
      #   branch:   master

• tgymnich/fork-sync and apps/pull: I don't want PRs. Besides there is not way to do a git rebase on GitHub website
• repo-sync/github-sync: It's not using rebase or merge. It completely mirrors the upstream so that it can't sync current branch
• wei/git-sync: Very complicated and have the same issue as github-sync. After all its aim is "syncing between two independent repo"
• aormsby/Fork-Sync-With-Upstream-action: If set git_pull_rebase_config: true, it's similar. But it tries to be configurable so that looks complex
• This one: Not widely tested. Use with caution