Disable the Link Manager on any future upgrade if it is still enabled…

… but there are no links. see #21307.

git-svn-id: http://core.svn.wordpress.org/trunk@21536 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/upgrade.php

@@ -402,6 +402,8 @@ function upgrade_all() {
 	if ( $wp_current_db_version < 21501 )
 		upgrade_350();
 
+	maybe_disable_link_manager();
+
 	maybe_disable_automattic_widgets();
 
 	update_option( 'db_version', $wp_db_version );
@@ -1908,9 +1910,7 @@ function wp_check_mysql_version() {
 }
 
 /**
- * {@internal Missing Short Description}}
- *
- * {@internal Missing Long Description}}
+ * Disables the Automattic widgets plugin, which was merged into core.
  *
  * @since 2.2.0
  */
@@ -1926,6 +1926,18 @@ function maybe_disable_automattic_widgets() {
 	}
 }
 
+/**
+ * Disables the Link Manager on upgrade, if at the time of upgrade, no links exist in the DB.
+ *
+ * @since 3.5.0
+ */
+function maybe_disable_link_manager() {
+	global $wp_current_db_version, $wpdb;
+
+	if ( $wp_current_db_version >= 21501 && get_option( 'link_manager_enabled' ) && ! $wpdb->get_var( "SELECT link_id FROM $wpdb->links LIMIT 1" ) )
+		update_option( 'link_manager_enabled', 0 );
+}
+
 /**
  * Runs before the schema is upgraded.
  *