Modify "changes is ready!" to "there is no change!" when appropriate

.github/workflows/doc-build.yml

@@ -170,6 +170,8 @@ jobs:
             # Restore the new doc dropping changes by "wipe out"
             (cd doc && git checkout --quiet -f HEAD~1)
             .github/workflows/create-changes-html.sh diff.txt doc
+            du -b diff.txt | cut -f1 > doc/DIFF_FILE_SIZE.txt
+            # note that the file above is generated after generating the diff, be careful to not contaminate the git diff
             # Sometimes rm -rf .git errors out because of some diehard hidden files
             # So we simply move it out of the doc directory
             (cd doc && mv .git ../git && mv .gitattributes ../gitattributes)

.github/workflows/doc-publish.yml

@@ -27,6 +27,7 @@ permissions:
 jobs:
   publish-doc:
     runs-on: ubuntu-latest
+    needs: doc-html
     if: github.event.workflow_run.conclusion == 'success'
     env:
       CAN_DEPLOY: ${{ secrets.NETLIFY_AUTH_TOKEN != '' &&  secrets.NETLIFY_SITE_ID != '' }}
@@ -52,6 +53,17 @@ jobs:
       - name: Extract doc
         run: unzip doc.zip -d doc
         if: steps.download-doc.outcome == 'success'
+        # WARNING: the folder structure is doc/doc/<files>, so for example the CHANGES.html
+        # generated by doc-build.yml is at doc/doc/CHANGES.html
+
+      - name: Read diff file size
+        id: read-diff-file-size
+        continue-on-error: true
+        run: |
+          cat doc/doc/DIFF_FILE_SIZE.txt  # debug print
+          { printf 'result='; cat doc/doc/DIFF_FILE_SIZE.txt; } >> "$GITHUB_OUTPUT"
+        # note that the result will have type string! although "== 0" coerce to number, see
+        # https://docs.github.com/en/actions/reference/workflows-and-actions/expressions
 
       - name: Deploy to Netlify
         id: deploy-netlify
@@ -75,7 +87,7 @@ jobs:
           header: preview-comment
           recreate: false
           message: |
-            [Documentation preview for this PR](${{ steps.deploy-netlify.outputs.NETLIFY_URL }}/html/en) (built with commit ${{ steps.source-run-info.outputs.sourceHeadSha }}; [changes](${{ steps.deploy-netlify.outputs.NETLIFY_URL }}/CHANGES.html)) is ready! :tada:
+            [Documentation preview for this PR](${{ steps.deploy-netlify.outputs.NETLIFY_URL }}/html/en) (built with commit ${{ steps.source-run-info.outputs.sourceHeadSha }}; ${{ steps.read-diff-file-size.outputs.result != 0 && format('[changes]({0}/CHANGES.html) is ready!', steps.deploy-netlify.outputs.NETLIFY_URL) || 'there is no change! ' }} :tada:
             This preview will update shortly after each push to this PR.
         if: steps.download-doc.outcome == 'success'