Cache get without set causes super long lock_waits with stampede protection

If you are using memcache, and I'm sure other types of alternative cache backends, there is an option for "stampede protection", which tries to ensure that several requests don't try to rebuild the same cache simultaneously, which may be an expensive operation for the system.

Metatag has an option to enabling caching, however it still attempts to read from the cache even when the setting for caching is disabled. This most prominently happens in metatag_page_build():

if ($cache = metatag_cache_get($cid)) {
      $metatags = $cache->data;
    }
    else {
      $metatags = metatag_metatags_view($instance, array());
      // If output caching is enabled, save this for later.
      if (variable_get('metatag_cache_output', FALSE)) {
        metatag_cache_set($cid, $metatags);
      }
    }

The problem is that with stampede protection on, the call to metatag_cache_get() will get a cache miss and then lock_wait for that cache item until the lock is released. Locks are released when the cache finally gets set, but since the cache setting is off, this doesn't happen.

In my particular case, we had enabled the setting for metatag caching, and then later disabled it. I think that could be the root cause of this whole thing - if the cache setting is never enabled in the first place, this may not be a problem, I'm not sure.

Here's a screenshot from New Relic of what the trace looks like when waiting:
https://imgur.com/a/9KVeccJ