Cache API: Handle numerically-keyed global_groups when switching sites.

This commit protects against another multisite cache edge-case where a persistent object-cache drop-in plugin (namely memcached) may use a numerically-keyed `global_groups` array instead of `'key' => true` like the default object-cache class, and includes the following changes:

* Use `wp_is_numeric_array()` inside of `wp_cache_switch_to_blog_fallback()` so that the global group names array are always properly formatted regardless of the caching back-end in use
* Add private helper methods to `Tests_Multisite_WpCacheSwitchToBlogFallback` to properly format global group names, and tweak a few tests to make them more resilient to different caching back-ends

Follow up to r61760.

See #23290.

git-svn-id: https://develop.svn.wordpress.org/trunk@61772 602fd350-edb4-49c9-b593-d223f7449a82

Author: JJJ

src/wp-includes/ms-blogs.php

@@ -614,10 +614,28 @@ function wp_cache_switch_to_blog_fallback() {
 	$global_groups         = false;
 	$non_persistent_groups = false;
 
-	if ( is_object( $wp_object_cache ) && isset( $wp_object_cache->global_groups ) ) {
-		$global_groups         = array_keys( $wp_object_cache->global_groups );
-		$all_groups            = array_fill_keys( array_keys( $wp_object_cache->cache ), true );
-		$non_persistent_groups = array_keys( array_diff_key( $all_groups, $wp_object_cache->global_groups ) );
+	if ( is_object( $wp_object_cache ) && isset( $wp_object_cache->global_groups ) && is_array( $wp_object_cache->global_groups ) ) {
+
+		// Get the global groups as they are.
+		$group_names = $wp_object_cache->global_groups;
+
+		// Get global group keys if non-numeric array.
+		if ( ! wp_is_numeric_array( $group_names ) ) {
+			$group_names = array_keys( $group_names );
+		}
+
+		$global_groups = $group_names;
+
+		/*
+		 * Non-persistent groups: Check for no_mc_groups first (memcached drop-in).
+		 * Fall back to cache structure (default cache).
+		 */
+		if ( isset( $wp_object_cache->no_mc_groups ) && is_array( $wp_object_cache->no_mc_groups ) && ! empty( $wp_object_cache->no_mc_groups ) ) {
+			$non_persistent_groups = $wp_object_cache->no_mc_groups;
+		} elseif ( isset( $wp_object_cache->cache ) && is_array( $wp_object_cache->cache ) ) {
+			$all_groups            = array_keys( $wp_object_cache->cache );
+			$non_persistent_groups = array_values( array_diff( $all_groups, $global_groups ) );
+		}
 	}
 
 	wp_cache_init();