Add branch sync reusable workflow

.github/workflows/branch-sync.yml

@@ -0,0 +1,119 @@
+# This is a reusable workflow, rather than an action
+
+name: Sync PR
+
+on:
+  workflow_call:
+    inputs:
+      head_branch:
+        type: string
+        required: false
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    env:
+      BASE_BRANCH: master
+      HEAD_BRANCH: ${{ inputs.head_branch || github.ref_name }}
+      STATUS_JSON: https://raw.githubusercontent.com/cylc/cylc-admin/master/docs/status/branches.json
+      FORCE_COLOR: 2
+    steps:
+      - name: Check branch name
+        shell: python
+        run: |
+          import os
+          import json
+          import sys
+          from urllib.request import urlopen
+
+          if os.environ['GITHUB_EVENT_NAME'] == 'schedule':
+            # Get branch from status page
+            meta = json.loads(
+              urlopen(os.environ['STATUS_JSON']).read()
+            )['meta_releases']
+            version = min(meta)
+            branch = meta[version][os.environ['GITHUB_REPOSITORY']]
+          else:
+            branch = os.environ['HEAD_BRANCH'].strip()
+
+          if branch.endswith('-sync'):
+            sys.exit("::error::Do not run this workflow for already-created sync branches")
+
+          with open(os.environ['GITHUB_ENV'], 'a') as F:
+            print(f'HEAD_BRANCH={branch}', file=F)
+            print(f'SYNC_BRANCH={branch}-sync', file=F)
+
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          ref: master
+
+      - name: Configure git
+        uses: cylc/release-actions/configure-git@v1
+
+      - name: Checkout sync branch if it exists
+        continue-on-error: true
+        run: |
+          git switch -c "$SYNC_BRANCH" "origin/${SYNC_BRANCH}"
+          echo "BASE_BRANCH=$SYNC_BRANCH" >> "$GITHUB_ENV"
+
+      - name: Attempt fast-forward
+        id: ff
+        run: |
+          if git merge "origin/${HEAD_BRANCH}" --ff-only; then
+            if [[ "$(git rev-parse HEAD)" == "$(git rev-parse "origin/${BASE_BRANCH}")" ]]; then
+              echo "::notice::$BASE_BRANCH is up to date with $HEAD_BRANCH"
+              exit 0
+            fi
+            git push origin "$BASE_BRANCH"
+          elif [[ "$BASE_BRANCH" == "$SYNC_BRANCH" ]]; then
+            echo "::notice::Cannot fast-forward $BASE_BRANCH to $HEAD_BRANCH; merge existing PR first"
+          else
+            echo "continue=true" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Attempt merge into master
+        id: merge
+        if: steps.ff.outputs.continue
+        run: |
+          git switch master
+          if git merge "origin/${HEAD_BRANCH}"; then
+            if git diff HEAD^ --exit-code --stat; then
+              echo "::notice::No diff between master and $HEAD_BRANCH"
+              exit 0
+            fi
+          else
+            git merge --abort
+          fi
+          echo "continue=true" >> $GITHUB_OUTPUT
+
+      - name: Push sync branch
+        id: push
+        if: steps.merge.outputs.continue
+        run: |
+          git switch -c "$SYNC_BRANCH" "origin/${HEAD_BRANCH}"
+          git push origin "$SYNC_BRANCH"
+          echo "continue=true" >> $GITHUB_OUTPUT
+
+      - name: Open PR
+        if: steps.push.outputs.continue
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          BODY: |
+            Please do a **normal merge**, not squash merge.
+            Please fix conflicts if necessary.
+
+            ---
+
+            Triggered by `${{ github.event_name }}`
+        run: |
+          url="$(
+            gh pr create --head "$SYNC_BRANCH" \
+            --title "🤖 Merge ${SYNC_BRANCH} into master" \
+            --body "$BODY"
+          )"
+          echo "::notice::PR created at ${url}"
+
+          gh pr edit "$SYNC_BRANCH" --add-label "sync" || true