1 files changed, 55 insertions, 3 deletions

diff --git a/engine/lib/upgrade.php b/engine/lib/upgrade.php
index f0874a483..158ec9ec1 100644
--- a/engine/lib/upgrade.php
+++ b/engine/lib/upgrade.php
@@ -17,8 +17,9 @@
  * @access private
  */
 function upgrade_code($version, $quiet = FALSE) {
+	// do not remove - upgrade scripts depend on this
 	global $CONFIG;
-
+	
 	$version = (int) $version;
 	$upgrade_path = elgg_get_config('path') . 'engine/lib/upgrades/';
 	$processed_upgrades = elgg_get_processed_upgrades();
@@ -244,7 +245,7 @@ function version_upgrade() {
 
 	// No version number? Oh snap...this is an upgrade from a clean installation < 1.7.
 	// Run all upgrades without error reporting and hope for the best.
-	// See http://trac.elgg.org/elgg/ticket/1432 for more.
+	// See https://github.com/elgg/elgg/issues/1432 for more.
 	$quiet = !$dbversion;
 
 	// Note: Database upgrades are deprecated as of 1.8.  Use code upgrades.  See #1433
@@ -291,7 +292,6 @@ function elgg_upgrade_bootstrap_17_to_18() {
 		'2011010101.php',
 	);
 
-	$upgrades_17 = array();
 	$upgrade_files = elgg_get_upgrade_files();
 	$processed_upgrades = array();
 
@@ -311,3 +311,55 @@ function elgg_upgrade_bootstrap_17_to_18() {
 
 	return elgg_set_processed_upgrades($processed_upgrades);
 }
+
+/**
+ * Creates a table {prefix}upgrade_lock that is used as a mutex for upgrades.
+ *
+ * @see _elgg_upgrade_lock()
+ *
+ * @return bool
+ * @access private
+ */
+function _elgg_upgrade_lock() {
+	global $CONFIG;
+	
+	if (!_elgg_upgrade_is_locked()) {
+		// lock it
+		insert_data("create table {$CONFIG->dbprefix}upgrade_lock (id INT)");
+		elgg_log('Locked for upgrade.', 'NOTICE');
+		return true;
+	}
+	
+	elgg_log('Cannot lock for upgrade: already locked.', 'WARNING');
+	return false;
+}
+
+/**
+ * Unlocks upgrade.
+ *
+ * @see _elgg_upgrade_lock()
+ *
+ * @access private
+ */
+function _elgg_upgrade_unlock() {
+	global $CONFIG;
+	delete_data("drop table {$CONFIG->dbprefix}upgrade_lock");
+	elgg_log('Upgrade unlocked.', 'NOTICE');
+}
+
+/**
+ * Checks if upgrade is locked
+ *
+ * @return bool
+ * @access private
+ */
+function _elgg_upgrade_is_locked() {
+	global $CONFIG;
+
+	$is_locked = count(get_data("show tables like '{$CONFIG->dbprefix}upgrade_lock'"));
+
+	// @todo why?
+	_elgg_invalidate_query_cache();
+
+	return $is_locked;
+}