Fail GitHub Actions CI build if code does not obey style rules

.github/workflows/collect_artifacts.yml

@@ -28,13 +28,49 @@ jobs:
         name: MuseScore-x86_64.AppImage
         path: ./build.release/MuseScore-x86_64.AppImage
 
-  collect_artifacts_MacOS:
+  codestyle_collect_artifacts_MacOS:
     runs-on: macos-latest
     steps:
     - name: Clone repository
       uses: actions/checkout@v2
       with:
         fetch-depth: 3
+    - name: Run code formatter
+    # Do this on MacOS because Homebrew has a more recent version of Uncrustify
+    # than is available in Ubuntu.
+      run: |
+        brew install uncrustify findutils
+        export PATH="/usr/local/opt/findutils/libexec/gnubin:${PATH}"
+        tools/codestyle/uncrustify_run.sh
+    - name: Test coding style
+      run: |
+        codestyle_failed_files="$(git diff --name-only)"
+        if [[ "${codestyle_failed_files}" ]]; then
+        cat >&2 <<EOF
+        Error: Code style is incorrect in these files:
+
+        ${codestyle_failed_files}
+
+        Please run tools/codestyle/uncrustify_run_file.sh on these files and
+        then amend your commit.
+
+        $ git show --name-only '*.h' '*.cpp' | xargs tools/codestyle/uncrustify_run_file.sh
+        $ git show --name-only '*.h' '*.cpp' | xargs git add
+        $ git commit --amend --no-edit
+
+        If your PR contains multiple commits then you must do an interactive
+        rebase and amend each commit in turn.
+
+        $ git -c sequence.editor='sed -i s/^pick/edit/' rebase -i HEAD~\${NUM_COMMITS}
+
+        Where \${NUM_COMMITS} is the number of commits in your PR.
+
+        The required changes are...
+
+        $(git diff)
+        EOF
+        exit 1
+        fi
     - name: get commit message
       run: |
          export commitmsg="$(git log --format=%B -n 1 --skip 1 | head -1)"