Cache API: Add wp_cache_flush_group function.

Add a new plugable function called `wp_cache_flush_group`, that will allow developers to clear whole cache groups with a single call. Developers can detect if their current implementation of an object cache supports flushing by group, by calling `wp_cache_supports_group_flush` which returns true if it is supported. If the developers of the object cache drop-in has not implemented `wp_cache_flush_group` and `wp_cache_supports_group_flush`, these functions are polyfilled and `wp_cache_supports_group_flush` defaults to false.

Props Spacedmonkey, filosofo, ryan, sc0ttkclark, SergeyBiryukov, scribu, Ste_95, dd32, dhilditch, dougal, lucasbustamante, dg12345, tillkruess, peterwilsoncc, flixos90, pbearne.
Fixes #4476.

git-svn-id: https://develop.svn.wordpress.org/trunk@53763 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/cache-compat.php

@@ -141,3 +141,49 @@ if ( ! function_exists( 'wp_cache_flush_runtime' ) ) :
 		return wp_using_ext_object_cache() ? false : wp_cache_flush();
 	}
 endif;
+
+if ( ! function_exists( 'wp_cache_flush_group' ) ) :
+	/**
+	 * Removes all cache items in a group, if the object cache implementation supports it.
+	 * Before calling this method, always check for group flushing support using the
+	 * `wp_cache_supports_group_flush()` method.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @see WP_Object_Cache::flush_group()
+	 * @global WP_Object_Cache $wp_object_cache Object cache global instance.
+	 *
+	 * @param string $group Name of group to remove from cache.
+	 * @return bool True if group was flushed, false otherwise.
+	 */
+	function wp_cache_flush_group( $group ) {
+		global $wp_object_cache;
+
+		if ( ! wp_cache_supports_group_flush() ) {
+			_doing_it_wrong(
+				__FUNCTION__,
+				__( 'Your object cache implementation does not support flushing individual groups.' ),
+				'6.1.0'
+			);
+
+			return false;
+		}
+
+		return $wp_object_cache->flush_group( $group );
+	}
+endif;
+
+if ( ! function_exists( 'wp_cache_supports_group_flush' ) ) :
+	/**
+	 * Whether the object cache implementation supports flushing individual cache groups.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @see WP_Object_Cache::flush_group()
+	 *
+	 * @return bool True if group flushing is supported, false otherwise.
+	 */
+	function wp_cache_supports_group_flush() {
+		return false;
+	}
+endif;