switch_to_blog() causing memory leak

I have not managed to fix the references issue, however when I implemented wp_cache_switch_to_blog things somewhat improve. Doing this means the object cache size (in php memory) will increase whenever switching to a _new_ blog, but restoring to older blogs has no overhead. So this happens:

1. wp_cache_init on blog 1 – memory used: 2mb
2. switch_to_blog( 2 ) – has an object cache of 1.5m – memory used: 3.5mb
3. restore current blog – memory used 3.5 (as blog 1 was already in the object cache)

If switching between a small amount of blogs this is acceptable (and better than this bug), however memory will still increase with the more blogs you switch to this way.

My implementation of wp_cache_switch_to_blog:

function wp_cache_switch_to_blog() {

	global $blog_id, $table_prefix, $wp_object_cache;
	$wp_object_cache->blog_prefix = ( is_multisite() ? $blog_id : $table_prefix ) . ':';
}

Ok, I have a fix!

Not sure what triggered this issue, I know switch_to_blog() did change, but looking at what changed I don’t see why this would have broken.

Anyhow, working on my above function, the key is to blow away the in-memory cache everytime switch to blog is called, this lets the memory footprint remain low no matter how many times you switch to different blogs:

if ( ! function_exists( 'wp_cache_switch_to_blog' ) ) :
function wp_cache_switch_to_blog() {

	global $blog_id, $table_prefix, $wp_object_cache;
	$wp_object_cache->cache = array();
	$wp_object_cache->blog_prefix = ( is_multisite() ? $blog_id : $table_prefix ) . ':';
}
endif;

To test this, I have a helper function to eat up memory:

function use_memory() {
	for( $i = 0; $i < 100; $i ++ ) {
		wp_cache_add( $i, str_pad( 'a', 1014 * 1014, 'a') );
	}
}

So to test (in init hook):

use_memory();
echo memory_get_usage() / 1024 / 1024; // should be about 120MB
switch_to_blog( 2 );
echo memory_get_usage() / 1024 / 1024; // should be about 25MB
restore_current_blog();
echo memory_get_usage() / 1024 / 1024; // should be about 120MB

You need to flush memcached every time running the test, so wp_cache_add() works

I am running the above function in production which fixed memory issues I was having, can now switch_blog_blog() 20+ times without issue