Add CI workflow to synchronize with shared repository labels

On every push that changes relevant files, and periodically, configure the repository's issue and pull request labels
according to the universal, shared, and local label configuration files.

Author: per1234

.github/label-configuration-files/labels.yml

@@ -0,0 +1,6 @@
+# Used by the "Sync Labels" workflow
+# See: https://github.com/Financial-Times/github-label-sync#label-config-file
+
+- name: "topic: preprocessor"
+  color: 00ffff
+  description: Related to sketch preprocessing

.github/workflows/sync-labels.yml

@@ -0,0 +1,133 @@
+# Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/sync-labels.md
+name: Sync Labels
+
+# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/sync-labels.ya?ml"
+      - ".github/label-configuration-files/*.ya?ml"
+  pull_request:
+    paths:
+      - ".github/workflows/sync-labels.ya?ml"
+      - ".github/label-configuration-files/*.ya?ml"
+  schedule:
+    # Run daily at 8 AM UTC to sync with changes to shared label configurations.
+    - cron: "0 8 * * *"
+  workflow_dispatch:
+  repository_dispatch:
+
+env:
+  CONFIGURATIONS_FOLDER: .github/label-configuration-files
+  CONFIGURATIONS_ARTIFACT: label-configuration-files
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Download JSON schema for labels configuration file
+        id: download-schema
+        uses: carlosperate/download-file-action@v1.0.3
+        with:
+          file-url: https://raw.githubusercontent.com/arduino/tooling-project-assets/main/workflow-templates/assets/sync-labels/arduino-tooling-gh-label-configuration-schema.json
+          location: ${{ runner.temp }}/label-configuration-schema
+
+      - name: Install JSON schema validator
+        run: |
+          sudo npm install \
+            --global \
+            ajv-cli \
+            ajv-formats
+
+      - name: Validate local labels configuration
+        run: |
+          # See: https://github.com/ajv-validator/ajv-cli#readme
+          ajv validate \
+            --all-errors \
+            -c ajv-formats \
+            -s "${{ steps.download-schema.outputs.file-path }}" \
+            -d "${{ env.CONFIGURATIONS_FOLDER }}/*.{yml,yaml}"
+
+  download:
+    needs: check
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        filename:
+          # Filenames of the shared configurations to apply to the repository in addition to the local configuration.
+          # https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/sync-labels
+          - universal.yml
+          - tooling.yml
+
+    steps:
+      - name: Download
+        uses: carlosperate/download-file-action@v1.0.3
+        with:
+          file-url: https://raw.githubusercontent.com/arduino/tooling-project-assets/main/workflow-templates/assets/sync-labels/${{ matrix.filename }}
+
+      - name: Pass configuration files to next job via workflow artifact
+        uses: actions/upload-artifact@v2
+        with:
+          path: |
+            *.yaml
+            *.yml
+          if-no-files-found: error
+          name: ${{ env.CONFIGURATIONS_ARTIFACT }}
+
+  sync:
+    needs: download
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Set environment variables
+        run: |
+          # See: https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#setting-an-environment-variable
+          echo "MERGED_CONFIGURATION_PATH=${{ runner.temp }}/labels.yml" >> "$GITHUB_ENV"
+
+      - name: Determine whether to dry run
+        id: dry-run
+        if: >
+          github.event == 'pull_request' ||
+          github.ref != format('refs/heads/{0}', github.event.repository.default_branch)
+        run: |
+          # Use of this flag in the github-label-sync command will cause it to only check the validity of the
+          # configuration.
+          echo "::set-output name=flag::--dry-run"
+
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Download configuration files artifact
+        uses: actions/download-artifact@v2
+        with:
+          name: ${{ env.CONFIGURATIONS_ARTIFACT }}
+          path: ${{ env.CONFIGURATIONS_FOLDER }}
+
+      - name: Remove unneeded artifact
+        uses: geekyeggo/delete-artifact@v1
+        with:
+          name: ${{ env.CONFIGURATIONS_ARTIFACT }}
+
+      - name: Merge label configuration files
+        run: |
+          # Merge all configuration files
+          shopt -s extglob
+          cat "${{ env.CONFIGURATIONS_FOLDER }}"/*.@(yml|yaml) > "${{ env.MERGED_CONFIGURATION_PATH }}"
+
+      - name: Install github-label-sync
+        run: sudo npm install --global github-label-sync
+
+      - name: Sync labels
+        env:
+          GITHUB_ACCESS_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          # See: https://github.com/Financial-Times/github-label-sync
+          github-label-sync \
+            --labels "${{ env.MERGED_CONFIGURATION_PATH }}" \
+            ${{ steps.dry-run.outputs.flag }} \
+            ${{ github.repository }}