Multisite: Introduce metadata for sites.

A new global multisite table `wp_blogmeta` is added to the database schema, and a set of `*_site_meta()` API functions are introduced.

The implementation fails gracefully when the new table is not yet available, which may happen especially shortly after the core update, before the network has been upgraded to the new database schema. The presence of the table is detected once and stored as a global setting on the main network.

Core does not yet use site metadata, but there are several use-cases to be implemented or explored in the near future, and it allows plugins to extend sites with arbitrary data, which will come in particularly handy with the upcoming REST API endpoint for sites.

Props spacedmonkey, johnjamesjacoby, jeremyfelt, flixos90.
Fixes #37923.

Built from https://develop.svn.wordpress.org/trunk@42836


git-svn-id: http://core.svn.wordpress.org/trunk@42666 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -4726,6 +4726,38 @@ function global_terms_enabled() {
 	return $global_terms;
 }
 
+/**
+ * Determines whether site meta is enabled.
+ *
+ * This function checks whether the 'blogmeta' database table exists. The result is saved as
+ * a setting for the main network, making it essentially a global setting. Subsequent requests
+ * will refer to this setting instead of running the query.
+ *
+ * @since 5.0.0
+ *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
+ * @return bool True if site meta is supported, false otherwise.
+ */
+function is_site_meta_supported() {
+	global $wpdb;
+
+	if ( ! is_multisite() ) {
+		return false;
+	}
+
+	$network_id = get_main_network_id();
+
+	$supported = get_network_option( $network_id, 'site_meta_supported', false );
+	if ( false === $supported ) {
+		$supported = $wpdb->get_var( "SHOW TABLES LIKE '{$wpdb->blogmeta}'" ) ? 1 : 0;
+
+		update_network_option( $network_id, 'site_meta_supported', $supported );
+	}
+
+	return (bool) $supported;
+}
+
 /**
  * gmt_offset modification for smart timezone handling.
  *