daethnir on "[Plugin: W3 Total Cache] Random Blank Pages - w3 total cache"

We ran into this as well. It's a bug in W3 Total Cache. I'm hoping the authors read this, else I'll try to get to them more directly.

We had this occurring on our brand-new wordpress install too. Turns out that W3TC caches HEAD and GET and serves them equally. So if a HEAD request for a URL is sent, it will be cached, and the next time a GET for that same resource is received it will return the cached HEAD response, which means you'll get a "Content-Length: 2" and a \r\n for your content, nothing else.

Here's a patch that will simply not cache HEAD at all:

diff --git wp-content/plugins/w3-total-cache/lib/W3/PgCache.php wp-content/plugins/w3-total-cache/lib/W3/PgCache.php
index aec6e61..44d0f61 100644
--- wp-content/plugins/w3-total-cache/lib/W3/PgCache.php
+++ wp-content/plugins/w3-total-cache/lib/W3/PgCache.php
@@ -493,20 +493,26 @@ class W3_PgCache {

         /**
          * Skip if posting
          */
         if ($_SERVER['REQUEST_METHOD'] == 'POST') {
             $this->cache_reject_reason = 'Requested method is POST';

             return false;
         }

+        if ($_SERVER['REQUEST_METHOD'] == 'HEAD') {
+            $this->cache_reject_reason = 'Requested method is HEAD';
+
+            return false;
+        }
+
         /**
          * Skip if there is query in the request uri
          */
         if (!$this->_config->get_boolean('pgcache.cache.query') && strstr($this->_request_uri, '?') !== false) {
             $this->cache_reject_reason = 'Requested URI contains query';

             return false;
         }

         /**

This is a large hammer that simply prevents any HEAD requests from being generated, so you never cache that 'blank' page. It does mean that you essentially have a heavy response generated each time for HEADs, so if caching is critical for your server load this is vulnerable to potential DoS.

A better solution when a HEAD request is made would be to generate the page as if a GET were requested and cache both the HEAD and GET responses independently and serve the appropriate one. I have not investigated the W3 Total Cache code enough to implement this.

I hope my change above, or the improved one I described, can make it into a updated version of the plugin. Please ping me if there are any questions.

Here's how to reproduce this:

• clear cache
• curl -I http://www.example.com/page/
  • You'll see just headers, as appropriate
• curl http://www.example.com/page/
  • You'll see just headers, which is wrong for a GET.