Multisite: Introduce get_current_network_id()

Similar to `get_current_blog_id`, this can be used to get the ID of the `$current_site` global. If not available, it will fallback to the main network ID. In single site, this will return 1.

Props spacedmonkey, flixos90.
Fixes #33900.

Built from https://develop.svn.wordpress.org/trunk@37670


git-svn-id: http://core.svn.wordpress.org/trunk@37636 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -4255,9 +4255,11 @@ function get_main_network_id() {
 		return 1;
 	}
 
+	$current_site = get_current_site();
+
 	if ( defined( 'PRIMARY_NETWORK_ID' ) ) {
 		$main_network_id = PRIMARY_NETWORK_ID;
-	} elseif ( 1 === (int) get_current_site()->id ) {
+	} elseif ( isset( $current_site->id ) && 1 === (int) $current_site->id ) {
 		// If the current network has an ID of 1, assume it is the main network.
 		$main_network_id = 1;
 	} else {

wp-includes/load.php

@@ -807,6 +807,29 @@ function get_current_blog_id() {
 	return absint($blog_id);
 }
 
+/**
+ * Retrieves the current network ID.
+ *
+ * @since 4.6.0
+ *
+ * @global WP_Network $current_site The current network.
+ *
+ * @return int The ID of the current network.
+ */
+function get_current_network_id() {
+	if ( ! is_multisite() ) {
+		return 1;
+	}
+
+	$current_site = get_current_site();
+
+	if ( ! isset( $current_site->id ) ) {
+		return get_main_network_id();
+	}
+
+	return absint( $current_site->id );
+}
+
 /**
  * Attempt an early load of translations.
  *

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-alpha-37669';
+$wp_version = '4.6-alpha-37670';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.