[wp-hackers] WP_Cache and contents filtering

[FUJINAKA Tohru]

Hi,

WP_Cache might be incompatible with contents filtering in some
situations.
I'm seeking any ideas to solve this problem.

I've installed Gengo, a plugin for make a wp site multilingual.
Visitors will see not all posts of the site but ones written in their
preferred language. That's one of Gengo's feature, site contents
filtering.

With WP_Cache, contents delivering flow could go wrong.
Let's look in how `wp_list_pages()' works.

--

These are 2 pages
  Id 1: About(en)
  Id 2: About(ja)

When the first visitor visits in en_US locale context, wp_list_pages()
begin fetching pages for output, calls get_pages().
Inside of get_pages() it once get all pages Id 1 and 2 but passes them
to filter handlers and Gengo will filter to leave only the Page [PageId:1].
Then get_pages() caches the result [PageId:1] into the storage.
As the result wp_list_pages() outputs a link of [PageId:1].

Next visitor comes, seems from ja_JP locale region.
wp_list_pages() calls get_pages().
get_pages() peeks cache and finds the subject, the previous call's
result [PageId:1]. Then dispatch filters and Gengo eliminates it because
[PageId:1] is not for the locale `ja' but for `en'.
As the result wp_list_pages() gets no page from get_pages(), so it
outputs no page link.

--

Similar behavior will be shown with categories and maybe others.
How can it be solved?

My ideas:

a. let plugins to implement functions alike get_pages and others just for
   manage caches.
   -> bad for maintenance: anti-patten `Copy and paste programming'.

b. make cache be structured.
   How about to setup cache storages for each locale, for example?
   The current cache implementation supports grouping. But its group
   name is hard-coded in general, you can see how get_pages() calls:
     wp_cache_get('get_pages', 'page').
   But with a bit of modification, you would be able to store a kind of
   information for each context.
   I got 2 ways in my mind:
     i)  make cache directory structured.
         Add a variable $subdir or something to WP_Object_Cache and
         add ways to set the value somehow.
         Directories `cache/en/', `cache/foo/bar/ja' would be made.
     ii) provide filters to manipulate a group name.
         Change some WP_Object_Cache's methods like get/set methods to
         call that filters.
         Group name `page' would be changed to 'page_en'.

   -> Caches can be retrieved anytime - not only after initialization of
      plugins but also before or middle of it.
      In this case cache structure could change gradually. It could be
      happened that fetching cache A from the directory 'cache/' then
      fetching cache A again but from 'cache/en/' then storing cache A
      into 'cache/en/foo/'.
   -> In addition, some variables are cached as global variables.
      One of them is `$alloptions', used by get_option() family.
      It will be hard to manage.

c. modify core code where manage caches to be locale aware.
   For instance, get_pages() will call wp_cache_get() like:

     $locale = get_locale();
     if ($cache = wp_cache_get('get_pages', "page_$locale"))
    
   -> It seems the best of the three to me if there is no other
      issue on cache.
      But is it true? Only for locale? Does it make sense?


Any opinions and/or ideas?

Regards,

--
  FUJINAKA Tohru aka reedom <tohru at nakarika.com>