When using switch_to_blog() with a persistent object cache that lacks wp_cache_switch_to_blog() support, non-persistent groups are not maintained

[markjaquith]

If you're using a persistent object cache that lacks the new wp_cache_switch_to_blog() support, WordPress core elses into a complete cache clobbering branch. It is smart about grabbing the global groups and re-adding those after wp_cache_init() is called, but it doesn't do the same for non-persistent groups. Those are a hardcoded array. So if you call switch_to_blog(), you would lose any custom-added non-persistent groups.

[nacin]

Curiously, not a regression — existed back when this was just a wp_cache_init() call too.

[ethitter]

@stevenkword and I worked on ​23290.patch during the Boston WP Hack Day.

We introduced a get_non_persistent_groups() method in Core's WP_Object_Cache to handle the core use, and to be implemented by authors of object cache drop ins. Initially we tried to include universal support for getting the non-persistent cache groups, but review of the APC Object Caching plugin revealed that the structure of $global_groups can vary between persistent backends.

The switching fallback that was present in switch_to_blog() and restore_current_blog() was abstracted into _ms_cache_switch_fallback() for consistency, and logic to use the persistent group getter closely mirrors the handling of global groups already in those functions.

[nacin]

I like this; let's address it in 3.7.

[wonderboymusic]

these are all marked 3.7-early

[nacin]

Let's make sure ryan looks this over.

[nunomorgadinho]

Any suggestions on what a unit test for this one should include? Maybe we could even separate it into different unit tests. I'm unsure and would appreciate feedback as I would be interested in working on it.

[wonderboymusic]

No unit tests - no Ryan blessing

[jeremyfelt]

We probably have an opportunity to get some unit tests in for this in 3.9.

@nunomorgadinho, if you are still interested in working on this, please feel free. It's probably worth looking at the current code in phpunit/tests/cache.php for examples of testing switch_to_blog(). We would then want to use additional tests for the pieces that this patch touches. Don't hesitate to provide any first attempts or stop in #wordpress-dev to go over approach.

[jeremyfelt]

This is probably going to miss 3.9 unless we can get some testing and tests in here.

[nacin]

There are a number of Cache API things we'd like to do, so moving this out of 3.9.

It's much, much better for object caches to introduce wp_cache_switch_to_blog(), so this is fairly low priority.