Fixes Elgg#4643. Prevent someone from running the upgrade script in p…

…arallel.

upgrade.php

@@ -20,6 +20,14 @@
 require_once(dirname(__FILE__) . "/engine/start.php");
 
 if (get_input('upgrade') == 'upgrade') {
+
+	// prevent someone from running the upgrade script in parallel (see #4643)
+	$upgrade_gatekeeper = elgg_get_data_path() . 'upgrading';
+	if (file_exists($upgrade_gatekeeper)) {
+		forward();
+	}
+	fclose(fopen($upgrade_gatekeeper, 'w'));
+
 	// disable the system log for upgrades to avoid exceptions when the schema changes.
 	elgg_unregister_event_handler('log', 'systemlog', 'system_log_default_logger');
 	elgg_unregister_event_handler('all', 'all', 'system_log_listener');
@@ -33,6 +41,10 @@
 	elgg_trigger_event('upgrade', 'system', null);
 	elgg_invalidate_simplecache();
 	elgg_reset_system_cache();
+
+	// critical region has past
+	unlink($upgrade_gatekeeper);
+
 } else {
 	// if upgrading from < 1.8.0, check for the core view 'welcome' and bail if it's found.
 	// see http://trac.elgg.org/ticket/3064
@@ -53,4 +65,4 @@
 	exit;
 }
 
-forward();
+forward();