Better caching for get_dirsize

[batmoo]

In a multisite install, when trying to determine whether a site has exceeded its storage quota, WordPress will scan through a blog's upload directory and sum up the file sizes, by running filesize against each one. With a large number of files, this can significantly slow down the upload process or certain portions of the Dashboard.

get_dirsize has transient caching in place but this is a single cache entry for all folders. It might be better if WordPress has a separate cache entry for each folder and was invalidated based on context so that get_dirsize does not need to be run constantly on older, unchanged directories as frequently.

We worked around this bug by making sure that upload_space_check_disabled was set; this resulted in an immediate speed-up.

My first thought would be to change recurse_dirsize() like so:

} elseif (is_dir($path)) {
	$handlesize = recurse_dirsize( $path );
	if ($handlesize > 0)
		$size += $handlesize;
}

to:

} elseif (is_dir($path)) {
	$handlesize = get_dirsize( $path );
	if ($handlesize > 0)
		$size += $handlesize;
}

This change would check the cache as it's doing the recursive iteration. This is very optimistic, though. Cache misses would result in a database SELECT and UPDATE/INSERT. For most sites, this change would probably slow things down. Further, this could result in a lot of extra rows in the db that wouldn't serve a useful purpose.

See ​http://mu.trac.wordpress.org/ticket/1175 for the origins of the get_dirsize() caching.

In [19747]:

Remove copy-pasted comment from upload_is_user_over_quota()'s phpdoc. It came from recurse_dirsize(). see #19879.