Add support for diffing the output when the build system changes

This is copied over from the website repo and aims to simplify
validation of build system changes, mostly dependency updates.

.github/workflows/render.yml

@@ -37,6 +37,140 @@ jobs:
         with:
           path: out/html
 
+  changes:
+    runs-on: ubuntu-latest
+    if: github.ref != 'refs/heads/main'
+    outputs:
+      build-system: ${{ steps.filter.outputs.build-system }}
+    steps:
+      - name: Check if build system changed
+        uses: dorny/paths-filter@v3
+        id: filter
+        with:
+          filters: |
+            build-system:
+              - .github/**/*
+              - package.json
+              - yarn.lock
+              - requirements.txt
+              - scripts/render*
+              - scripts/setup.sh
+
+  build-main:
+    needs:
+      - changes
+    if: needs.changes.outputs.build-system == 'true'
+    env:
+      # ruby/setup-ruby@v1 does not offer a way to change the cached gems path.
+      # See https://github.com/ruby/setup-ruby/issues/291
+      GLOBAL_GEMS: 1
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: main
+
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version-file: '.python-version'
+
+      - name: Install dependencies
+        run: bash ./scripts/setup.sh
+
+      - uses: actions/setup-node@v3
+        with:
+          node-version-file: '.node-version'
+
+      - name: Install
+        run: yarn install
+
+      - name: Build newsletters
+        run: bash ./scripts/render-all.sh
+
+      - name: Upload baseline
+        uses: actions/upload-pages-artifact@v2
+        with:
+          name: build-from-main
+          path: out/html
+
+  diff:
+    runs-on: ubuntu-latest
+    needs:
+      - build
+      - build-main
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Download artifact from this branch's build
+        uses: actions/download-artifact@v3
+        with:
+          name: github-pages
+
+      - name: Unpack local build
+        run: |
+          mkdir local
+          pushd local
+          tar --extract --file ../artifact.tar
+          popd
+          mv artifact.tar local.tar
+
+      - name: Download artifact from main branch build
+        uses: actions/download-artifact@v3
+        with:
+          name: build-from-main
+          path: main
+
+      - name: Replace build timestamps
+        run: sed -i -r '1 s_<updated>[^/]+</updated>_<updated>NOW</updated>_' {main,local}/feed.xml
+
+      - name: Diff
+        id: diff
+        run: |
+          set +e
+          diff -ru main local > ./result.diff
+          result=$?
+          set -euo pipefail
+
+          delimiter="gha-delim-$RANDOM-$RANDOM-gha-delim"
+          {
+            echo "diff<<${delimiter}"
+            cat ./result.diff
+            echo "${delimiter}"
+          } >> "$GITHUB_OUTPUT"
+
+          cat ./result.diff
+
+          if [[ $result -ne 0 ]]
+          then
+            echo has-changes=true >> "$GITHUB_OUTPUT"
+          else
+            echo has-changes=false >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Build comment
+        id: build-comment
+        if: always()
+        run: |
+          {
+            if [[ "${{ steps.diff.outputs.has-changes }}" = "true" ]]
+            then
+              echo "🔀 Build diff:"
+              echo '```diff'
+              cat ./result.diff
+              echo '```'
+            else
+              echo "Build has no changes."
+            fi
+          } > ./comment.md
+
+      - name: Post diff to PR
+        uses: thollander/actions-comment-pull-request@v2
+        if: always()
+        with:
+          comment_tag: build-diff
+          filePath: ./comment.md
+
   deploy:
     name: Deploy to GitHub Pages
     runs-on: ubuntu-latest