[wp-hackers] memcached object cache breaking http head requests

[Casey Bisson]

This is an odd one.

When running WordPress MU (tested in 2.8x and 2.9x) with the memcached
object cache active, it refuses to respond to HTTP HEAD requests. The result
of this is that head requests to check the mimetype of a linked file (as for
setting the enclosure) or size (as the video framework plugin does) fail.

curl -I http://url.path


returns either an empty result, or (if fronted with varnish) a 503 error.

telnet site.org 80



HEAD /url.path HTTP/1.1

host: site.org


returns the same.

Moving object-cache.php out of /wp-content or commenting out one line (
$this->mc[$bucket]->addServer(...) ) solves the problem. Oddly, the problem
occurs only when wp_cache_init() is called the second time in
wp-settings.php (if I comment out that line, the problem disappears as well,
even though the cache was initialized previously).

The problem occurs with multiple revisions memcache object-cache.php,
including the most recent, on multiple servers with different versions of
memcached.
http://plugins.trac.wordpress.org/browser/memcached/trunk/

I have not yet had an opportunity to test this with non-MU versions of WP or
with /trunk/ (where it appears that calls to wp_cache_init() have been
replaced with wp_start_object_cache() ), so there's a good chance this is MU
specific or fixed already, but in all my searching I haven't been able to
find any mention of the problem.

I'd love to know if others have experienced this problem or solved it.

--Casey