Add GitHub workflow for syncing with upstream repository

Author: tsiakoulias

.github/workflows/sync-upstream.yml

@@ -0,0 +1,68 @@
+name: Sync from upstream
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "17 * * * *"
+
+permissions:
+  contents: write
+  pull-requests: write
+
+concurrency:
+  group: sync-upstream
+  cancel-in-progress: false
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    env:
+      UPSTREAM_REPO: ReactiveDrop/reactivedrop_public_src
+      SYNC_BRANCH: chore/sync-upstream
+    steps:
+      - name: Checkout fork
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Sync branch with upstream
+        id: sync
+        env:
+          BASE_BRANCH: ${{ github.event.repository.default_branch }}
+        run: |
+          set -euo pipefail
+
+          git remote add upstream "https://github.com/${UPSTREAM_REPO}.git"
+          git fetch origin "$BASE_BRANCH"
+          git fetch upstream "$BASE_BRANCH" --prune
+
+          upstream_ahead_count="$(git rev-list --count "origin/$BASE_BRANCH..upstream/$BASE_BRANCH")"
+          if [ "$upstream_ahead_count" -eq 0 ]; then
+            echo "has_updates=false" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          git checkout -B "$SYNC_BRANCH" "upstream/$BASE_BRANCH"
+          git push --force-with-lease origin "$SYNC_BRANCH"
+          echo "has_updates=true" >> "$GITHUB_OUTPUT"
+
+      - name: Create pull request
+        if: steps.sync.outputs.has_updates == 'true'
+        env:
+          BASE_BRANCH: ${{ github.event.repository.default_branch }}
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          set -euo pipefail
+
+          existing_pr="$(gh pr list --state open --head "$SYNC_BRANCH" --base "$BASE_BRANCH" --json number --jq '.[0].number')"
+          if [ -n "$existing_pr" ]; then
+            echo "PR #$existing_pr already open."
+            exit 0
+          fi
+
+          gh pr create \
+            --base "$BASE_BRANCH" \
+            --head "$SYNC_BRANCH" \
+            --title "Sync from upstream" \
+            --body "Automated sync from upstream."
+