_get_last_post_time() uses the incorrect cache key

[mdawaffe]

[15838] sets the cache key to

"lastpost{$field}:" . get_current_blog_id() . ":$timezone:" . md5( serialize( $post_types ) )

but that key is never invalidated. The invalidation is meant to be handled by _transition_post_status().

There's no need to explicitly include the blog_id in a cache key.

If the data needs to be cached by some dynamic variable ($post_types, in this case), there are a few options.

1. The invalidation needs to be a lot smarter (hard).
2. The cached data needs to be an array ($data[ serialize( ... ) ] = $date) so that it can be invalidated all at once (bad - leads to race conditions).
3. The data should only be cached for one single page load (wp_cache_add_non_persistent_groups()).
4. Something else?

Is there a way to invalidate the entire 'timeinfo' group?

A compromise would be to use a single cache key for all post types. That will still probably take care of the original problem.

Going to close as dup of #14922, to keep everything in one place.

Replying to scribu:

Is there a way to invalidate the entire 'timeinfo' group?

No. Memcache (and most wp_cache implementations) cannot do this.

There's a way around this, but it's tricky. You keep track of an incrementor, $i, in the cache and then do cache operations on the group "timeinfo-$i". To "invalidate" the group, you increment $i using wp_cache_incr().

Replying to scribu:

A compromise would be to use a single cache key for all post types. That will still probably take care of the original problem.

That would work.

[16537]