Random Blank Pages

Do W3TC and Quick Cache share some of the same code, or are they fully independent implementations?

These are separate applications. However, they do both share WordPress in common of course; and they both use the advanced-cache.php drop-in functionality provided by the WordPress core. Otherwise though, they are not sharing the same PHP code; these plugins are developed by two different entities. Quick Cache having been developed by WebSharks, Inc.

I’m not sure how to reproduce it

I would try to identify which particular portions of the raw HTML code are not making it into the final presentation. Is it possible that some portions of your site are powered by sessions? Or, some other unique identifier that will only work for the session under which the file was originally cached?

I’ve noticed the issue happens with WordPress posts, and visitors that get a cache version of the page, so I doubt there are sessions. I’ll paste a copy of the full and incomplete post source via pastebin.com, once it happens again.

I’ve done some more checks
In cache directory
ls -l | wc -l
2382

Nearly 2400 cached file.
grep -L body qc-c* | wc -l
23

That’s 23 cached files without body. That’s just about 1% of the pages and posts, but still annoying. I guess I could write a script to delete cached files without body regularly, as a temporary hack. Would that cause problems with quick cache?

I checked one of the problematic pages.
Without body: https://pastebin.com/v1F8tGmX
After clearing the cache: https://pastebin.com/i0tHQmnm

The full header is generated in both files (until line 110), but there’s no body at all in the other, except a script probably added by cloudflare, and the </html> is also missing.

For W3TC, some people said it could be because it’s caching HTTP HEAD requests.

Thanks for those extra details. Which theme are you running please?

I’m still running Inove…
That’s the site: cnx-software.com

Just one more thing. That’s a ugly solution, but it works.
I’ve written a script to delete cache files without body.

Here it is: https://pastebin.com/24Mav7dM
You just need to set WP_PATH to your WordPress installation path.

I’ve setup a cron job to run it regularly on my VPS.

Thank you for the follow-ups.

I tested this against iNove but I was unable to reproduce it in a short round of tests. That said, I suspect the underlying cause of this could be related to PHP parse errors under certain conditions; which may result in error-ridden pages in your cache; which are missing the body.

To confirm this, I suggest that you enable PHP error logging (or ask your hosting company to do this for you); then begin monitoring your PHP error log for any clues that might lead you to the underlying cause.

I’m not seeing a connection between this and the Quick Cache plugin at the present time however. If you have any further details you can provide in the future please let us know. Thanks!

I can’t see anything in fpm-php5.log, but somebody noticed the problem appears to go away when switching from nginx to apache.