fix(nginx): browsers revalidate static files

Static URLs carry no version, but nginx served them with expires max, so
after an update or a theme change browsers kept the old CSS and JS. They
now revalidate each load (304 when unchanged). CI checks the header, the
304 and that a changed file is sent again.

.github/workflows/test.yml

@@ -156,6 +156,24 @@ jobs:
       - name: Import Compiled Dependencies
         run: docker exec wvc /srv/webvirtcloud/venv/bin/python3 -c "import libvirt, ldap, lxml.etree, psycopg, sass"
 
+      # Static URLs carry no version: browsers must revalidate them
+      - name: Static Files Revalidated
+        run: |
+          url=http://127.0.0.1:8080/static/css/wvc-main.min.css
+          header() { tr -d '\r' | awk -v h="$1" 'tolower($0) ~ "^" h ":" {sub(/^[^:]*: */, ""); print}'; }
+          h=$(curl -s -m 5 -D - -o /dev/null "$url")
+          [ "$(echo "$h" | header cache-control)" = no-cache ] || { echo "$h"; exit 1; }
+          etag=$(echo "$h" | header etag)
+          [ -n "$etag" ]
+          h=$(curl -s -m 5 -D - -o /dev/null -H "If-None-Match: $etag" "$url")
+          echo "$h" | head -1 | grep -q " 304" || { echo "$h"; exit 1; }
+          [ "$(echo "$h" | header cache-control)" = no-cache ]
+          docker exec wvc sh -c 'echo "/* changed */" >> /srv/webvirtcloud/staticfiles/css/wvc-main.min.css'
+          h=$(curl -s -m 5 -D - -o /dev/null -H "If-None-Match: $etag" "$url")
+          echo "$h" | head -1 | grep -q " 200" || { echo "$h"; exit 1; }
+          new=$(echo "$h" | header etag)
+          [ -n "$new" ] && [ "$new" != "$etag" ]
+
       # The browser reaches the console through nginx on the panel's port
       - name: Console Through Nginx Only
         run: |