6 files changed, 102 insertions, 7 deletions
diff --git a/actions/admin/site/unlock_upgrade.php b/actions/admin/site/unlock_upgrade.php
new file mode 100644
index 000000000..b625b1d26
--- /dev/null
+++ b/actions/admin/site/unlock_upgrade.php
@@ -0,0 +1,10 @@
+<?php
+/**
+ * Unlocks the upgrade script 
+ */
+
+if (_elgg_upgrade_is_locked()) {
+	_elgg_upgrade_unlock();
+}
+system_message(elgg_echo('upgrade:unlock:success'));
+forward(REFERER);
diff --git a/engine/lib/admin.php b/engine/lib/admin.php
index 3f23f079c..cb9524f11 100644
--- a/engine/lib/admin.php
+++ b/engine/lib/admin.php
@@ -233,6 +233,7 @@ function admin_init() {
 	elgg_register_action('admin/site/update_basic', '', 'admin');
 	elgg_register_action('admin/site/update_advanced', '', 'admin');
 	elgg_register_action('admin/site/flush_cache', '', 'admin');
+	elgg_register_action('admin/site/unlock_upgrade', '', 'admin');
 
 	elgg_register_action('admin/menu/save', '', 'admin');
 
diff --git a/engine/lib/upgrade.php b/engine/lib/upgrade.php
index f0874a483..f4f4b16f5 100644
--- a/engine/lib/upgrade.php
+++ b/engine/lib/upgrade.php
@@ -311,3 +311,58 @@ 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, $DB_QUERY_CACHE;
+	
+	$is_locked = count(get_data("show tables like '{$CONFIG->dbprefix}upgrade_lock'"));
+	
+	// Invalidate query cache
+	if ($DB_QUERY_CACHE) {
+		$DB_QUERY_CACHE->clear();
+		elgg_log("Query cache invalidated", 'NOTICE');
+	}
+	
+	return $is_locked;
+}
diff --git a/languages/en.php b/languages/en.php
index 2b5ba00fd..f3acc50ee 100644
--- a/languages/en.php
+++ b/languages/en.php
@@ -1055,6 +1055,10 @@ Once you have logged in, we highly recommend that you change your password.
 	'upgrading' => 'Upgrading...',
 	'upgrade:db' => 'Your database was upgraded.',
 	'upgrade:core' => 'Your Elgg installation was upgraded.',
+	'upgrade:unlock' => 'Unlock upgrade',
+	'upgrade:unlock:confirm' => "The database is locked for another upgrade. Running concurrent upgrades is dangerous. You should only continue if you know there is not another upgrade running. Unlock?",
+	'upgrade:locked' => "Cannot upgrade. Another upgrade is running. To clear the upgrade lock, visit the Admin section.",
+	'upgrade:unlock:success' => "Upgrade unlocked suscessfully.",
 	'upgrade:unable_to_upgrade' => 'Unable to upgrade.',
 	'upgrade:unable_to_upgrade_info' =>
 		'This installation cannot be upgraded because legacy views
diff --git a/upgrade.php b/upgrade.php
index 60764ba93..c5f158c61 100644
--- a/upgrade.php
+++ b/upgrade.php
@@ -9,6 +9,8 @@
  * new version of the script. Deleting the script is not a requirement and
  * leaving it behind does not affect the security of the site.
  *
+ * Upgrades use a table {db_prefix}upgrade_lock as a mutex to prevent concurrent upgrades.
+ *
  * @package Elgg.Core
  * @subpackage Upgrade
  */
@@ -20,6 +22,12 @@ define('UPGRADING', 'upgrading');
 require_once(dirname(__FILE__) . "/engine/start.php");
 
 if (get_input('upgrade') == 'upgrade') {
+	// prevent someone from running the upgrade script in parallel (see #4643)
+	if (!_elgg_upgrade_lock()) {
+		register_error(elgg_echo('upgrade:locked'));
+		forward();
+	}
+	
 	// 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 +41,9 @@ if (get_input('upgrade') == 'upgrade') {
 	elgg_trigger_event('upgrade', 'system', null);
 	elgg_invalidate_simplecache();
 	elgg_reset_system_cache();
+	
+	_elgg_upgrade_unlock();
+	
 } 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 +64,4 @@ if (get_input('upgrade') == 'upgrade') {
 	exit;
 }
 
-forward();
-\ No newline at end of file
+forward();
diff --git a/views/default/widgets/control_panel/content.php b/views/default/widgets/control_panel/content.php
index d2db54bc6..a348d612f 100644
--- a/views/default/widgets/control_panel/content.php
+++ b/views/default/widgets/control_panel/content.php
@@ -11,12 +11,26 @@ elgg_register_menu_item('admin_control_panel', array(
 	'link_class' => 'elgg-button elgg-button-action',
 ));
 
-elgg_register_menu_item('admin_control_panel', array(
-	'name' => 'upgrade',
-	'text' => elgg_echo('upgrade'),
-	'href' => 'upgrade.php',
-	'link_class' => 'elgg-button elgg-button-action',
-));
+// @todo Move in this in ElggUpgradeManager::isLocked() when #4682 fixed
+$is_locked = _elgg_upgrade_is_locked();
+
+if (!$is_locked) {
+	elgg_register_menu_item('admin_control_panel', array(
+		'name' => 'upgrade',
+		'text' => elgg_echo('upgrade'),
+		'href' => 'upgrade.php',
+		'link_class' => 'elgg-button elgg-button-action',
+	));
+} else {
+	elgg_register_menu_item('admin_control_panel', array(
+		'name' => 'unlock_upgrade',
+		'text' => elgg_echo('upgrade:unlock'),
+		'href' => 'action/admin/site/unlock_upgrade',
+		'is_action' => true,
+		'link_class' => 'elgg-button elgg-button-action',
+		'confirm' => elgg_echo('upgrade:unlock:confirm'),
+	));
+}
 
 echo elgg_view_menu('admin_control_panel', array(
 	'class' => 'elgg-menu-hz',