--- a/web/wp-cron.php	Thu Sep 16 15:45:36 2010 +0000
+++ b/web/wp-cron.php	Mon Nov 19 18:26:13 2012 +0100
@@ -1,7 +1,7 @@
 <?php
 /**
  * WordPress Cron Implementation for hosts, which do not offer CRON or for which
- * the user has not setup a CRON job pointing to this file.
+ * the user has not set up a CRON job pointing to this file.
  *
  * The HTTP request to this file will not slow down the visitor who happens to
  * visit when the cron job is needed to run.
@@ -22,39 +22,83 @@
 define('DOING_CRON', true);
 
 if ( !defined('ABSPATH') ) {
-	/** Setup WordPress environment */
+	/** Set up WordPress environment */
 	require_once('./wp-load.php');
 }
 
+// Uncached doing_cron transient fetch
+function _get_cron_lock() {
+	global $_wp_using_ext_object_cache, $wpdb;
+
+	$value = 0;
+	if ( $_wp_using_ext_object_cache ) {
+		// Skip local cache and force refetch of doing_cron transient in case
+		// another processs updated the cache
+		$value = wp_cache_get( 'doing_cron', 'transient', true );
+	} else {
+		$row = $wpdb->get_row( $wpdb->prepare( "SELECT option_value FROM $wpdb->options WHERE option_name = %s LIMIT 1", '_transient_doing_cron' ) );
+		if ( is_object( $row ) )
+			$value = $row->option_value;
+	}
+
+	return $value;
+}
+
 if ( false === $crons = _get_cron_array() )
 	die();
 
 $keys = array_keys( $crons );
-$local_time = time();
+$local_time = microtime( true );
 
 if ( isset($keys[0]) && $keys[0] > $local_time )
 	die();
 
-foreach ($crons as $timestamp => $cronhooks) {
+$doing_cron_transient = get_transient( 'doing_cron');
+
+// Use global $doing_wp_cron lock otherwise use the GET lock. If no lock, trying grabbing a new lock.
+if ( empty( $doing_wp_cron ) ) {
+	if ( empty( $_GET[ 'doing_wp_cron' ] ) ) {
+		// Called from external script/job. Try setting a lock.
+		if ( $doing_cron_transient && ( $doing_cron_transient + WP_CRON_LOCK_TIMEOUT > $local_time ) )
+			return;
+		$doing_cron_transient = $doing_wp_cron = sprintf( '%.22F', microtime( true ) );
+		set_transient( 'doing_cron', $doing_wp_cron );
+	} else {
+		$doing_wp_cron = $_GET[ 'doing_wp_cron' ];
+	}
+}
+
+// Check lock
+if ( $doing_cron_transient != $doing_wp_cron )
+	return;
+
+foreach ( $crons as $timestamp => $cronhooks ) {
 	if ( $timestamp > $local_time )
 		break;
 
-	foreach ($cronhooks as $hook => $keys) {
+	foreach ( $cronhooks as $hook => $keys ) {
 
-		foreach ($keys as $k => $v) {
+		foreach ( $keys as $k => $v ) {
 
 			$schedule = $v['schedule'];
 
-			if ($schedule != false) {
+			if ( $schedule != false ) {
 				$new_args = array($timestamp, $schedule, $hook, $v['args']);
 				call_user_func_array('wp_reschedule_event', $new_args);
 			}
 
-			wp_unschedule_event($timestamp, $hook, $v['args']);
+			wp_unschedule_event( $timestamp, $hook, $v['args'] );
+
+ 			do_action_ref_array( $hook, $v['args'] );
 
- 			do_action_ref_array($hook, $v['args']);
+			// If the hook ran too long and another cron process stole the lock, quit.
+			if ( _get_cron_lock() != $doing_wp_cron )
+				return;
 		}
 	}
 }
 
+if ( _get_cron_lock() == $doing_wp_cron )
+	delete_transient( 'doing_cron' );
+
 die();