GHA automation to merge code from jdk21u-dev

.github/workflows/merge-from-upstream-tip.yml

@@ -0,0 +1,54 @@
+name: "Merge Latest Tag from Tip"
+on:
+  schedule:
+    - cron: '0 14 * * 4'
+  workflow_dispatch:
+jobs:
+  # To create personal access tokens, go to account settings,
+  # then developer settings. Create a legacy token, give it
+  # access to repos and workflows and other sensible things.
+  # To then create a secret for the workflow, go to the
+  # repository settings and create a new secret with the text
+  # of the access token.
+  merge-from-tip:
+    # We only want this to run in one repo
+    # if: github.repository == 'openjdk/shenandoah'
+    runs-on: ubuntu-latest
+    name: Merge latest tag
+    steps:
+      # Note that we are checking out the upstream fork here.
+      # If this automation runs in openjdk/shenandoah, remove
+      # the token and repository arguments.
+      - name: Checkout code
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+          token: ${{ secrets.WORKFLOWS }}
+          repository: openjdk/shenandoah-jdk21u
+      - name: Fetch latest tag
+        id: latest-tag
+        run: |
+          git remote add upstream https://github.com/openjdk/jdk21u-dev.git
+          git fetch upstream
+          echo "latest=$(git tag --list --sort=-creatordate | head -1)" >> "$GITHUB_OUTPUT"
+      - name: Reset the default branch with latest tag
+        run: |
+          git reset --hard ${{ steps.latest-tag.outputs.latest }}
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@v5
+        with:
+          commit-message: Merge tag ${{ steps.latest-tag.outputs.latest }}
+          # Using the legacy personal access token seems to be
+          # required to create a pull request on a remote repo.
+          # We can revert this to a fine grained token when this
+          # workflow is running upstream.
+          token: ${{ secrets.PAT }}
+          branch: merge-${{ steps.latest-tag.outputs.latest }}
+          delete-branch: true
+          # Without this, this action will attempt to push the
+          # new branch to the remote fork (i.e. openjdk/shenandoah).
+          # When this workflow runs in that repo, we can remove this.
+          push-to-fork: ${{ github.repository }}
+          title: Merge openjdk/jdk21u-dev:master
+          body: Merges tag ${{ steps.latest-tag.outputs.latest }}
+