[ticket/17394] Check mergeability of 3.3.x PRs

PHPBB-17394

.github/workflows/tests.yml

@@ -596,3 +596,54 @@ jobs:
               if: ${{ matrix.type == 'functional' }}
               run: |
                   phpBB/vendor/bin/phpunit --configuration .github/phpunit-psql-windows-github.xml --verbose --stop-on-error --group functional
+
+    merge-check:
+        if: github.event.pull_request.base.ref == '3.3.x'
+        runs-on: ubuntu-latest
+        steps:
+            -   name: Checkout the repository
+                uses: actions/checkout@v3
+                with:
+                    fetch-depth: 0  # Ensure full history is fetched
+
+            -   name: Set up Git user
+                run: |
+                    git config --global user.name "github-actions"
+                    git config --global user.email "github-actions@github.com"
+
+            -   name: Fetch all branches
+                run: git fetch origin
+
+            -   name: Simulate merging PR into 3.3.x
+                id: simulate_merge
+                run: |
+                    git checkout 3.3.x
+                    git merge --no-ff ${{ github.event.pull_request.head.sha }} --no-commit --no-ff || exit 1
+
+            -   name: Attempt to merge updated 3.3.x into master
+                id: merge_master
+                run: |
+                    git checkout master
+                    if git merge --no-ff origin/3.3.x --no-commit --no-ff; then
+                      echo "::set-output name=mergeable::true"
+                    else
+                      echo "::set-output name=mergeable::false"
+                      git merge --abort
+                    fi
+
+            -   name: Post comment on PR
+                if: always()  # Ensure this step always runs, regardless of merge result
+                uses: peter-evans/create-or-update-comment@v4
+                with:
+                    token: ${{ secrets.GITHUB_TOKEN }}
+                    issue-number: ${{ github.event.pull_request.number }}
+                    body: |
+                        The attempt to merge branch `3.3.x` into `master` has completed after considering the changes in this PR.
+
+                        - Merge result: ${{ steps.merge_master.outputs.mergeable == 'true' && 'Success' || 'Conflict' }}
+
+                        If there is a conflict, a separate PR will be needed to merge `3.3.x` into `master`.
+
+            -   name: Mark job as succeeded
+                if: always()
+                run: echo "Merge check completed. Ignoring the result to avoid failed status."