Invalid $blogcacheid value for multisite installs within a subdirectory

Hello,

we have a wordpress multisite installation within a subdirectory which results in urls like https://domain/blog/blog1/ and https://domain/blog/blog2 and so on.

The following code is located in wp-cache-base.php and assigns the $blogcacheid variable with values like "blog/blog1" or "blog/blog2".

`if ( is_object( $current_blog ) && function_exists( 'is_subdomain_install' ) ) {
$blogcacheid = is_subdomain_install() ? $current_blog->domain : trim( $current_blog->path, '/' );
}`

In wp-cache-phase2.php the built cache files are then renamed to their final name which contain the value of $blogcacheid.
This fails if the value contains a slash and the debug log shows an error like "FAILED to rename temp wp-cache file to {$dir}{$cache_filename}".

Because of this bug the cache is not working for feeds. It also causes an ever growing number of temporary cache files in the wp-content/cache folder since the temp files won't get deleted when the renaming fails.

Start in wp-cache-base.php and wp-cache-phase2.php, tracing how a subdirectory multisite value is assigned to $blogcacheid and then included in the final cache filename. Reproduce the issue with a subdirectory multisite install, checking feed caching and the rename failure. Done means cache files can be finalized without accumulating temporary files and feeds are cached.

Written by the indexing model from the issue text.