PageCache: async rendering of blocks can corrupt layout cache #8554
Description
Hi folks,
I disovered a potential bug regarding the layout cache which leads to a blank page. If this happens, customers may see a white page without any hmtl inside the body tags. It might be related to #6942, although there is no php-fpm or libxml in the game here.
Preconditions
- Tested on Magento 2.1.0 CE and 2.1.4 CE
Steps to reproduce
bin/magento cache:cleancurl -gI "http://magento2.local/page_cache/block/render/?ajax=1&blocks=[%22header.login%22]&handles=[%22default%22%2C%22cms_index_index%22%2C%22cms_page_view%22%2C%22cms_index_index_id_home%22]&originalRequest=a&version="curl -s http://magento2.local | grep "<body" -A 3 -B 3
Expected result
- Body tag should still contain some data
Actual result
- Generated DOM has an empty body like this
<body data-container="body" data-mage-init='{"loaderAjax": {}, "loader": { "icon": "http://magento2.local/pub/static/version1487104262/frontend/Magento/luma/de_DE/images/loader-2.gif"}}' class="cms-index-index page-layout-1column">
</body>
Discussion
This ajax call seems to destroy the layout cache. If it is not present, structure which gets loaded and unserialized in lib/internal/Magento/Framework/View/Layout.php:generateElements from cache does not contain any root element anymore:
$ awk -F "root" '{print NF-1}' var/cache/mage--0/mage---792_STRUCTURE_LAYOUT_FRONTEND_STORE1_2C7461AF04BB11FE209659D7147D2C579
0
0
Cleaning the cache again and loading the front page normally generates a valid layout cache, where root is present:
$ awk -F "root" '{print NF-1}' var/cache/mage--0/mage---792_STRUCTURE_LAYOUT_FRONTEND_STORE1_2C7461AF04BB11FE209659D7147D2C579
0
98
I think it has something do to with the used handles [default, cms_index_index, cms_page_view, cms_index_index_id_home] which are
used for generating the cache key in getCacheId. If there is no cache present, the page cache block just writes its content into the layout cache although there is no root layout used.
I need to dive deeper into this issue. It normally should not happen, because the DOM should get loaded first. And subsequent ajax calls should always have a layout cache present. But if cache gets cleared manually and there is already an ajax call on its way it can lead to a broken layout cache.