- Patch #561990 by catch, mikeytown2, nnewton: avoid variable_set() and variable_del() stampedes.

includes/bootstrap.inc

@@ -743,13 +743,26 @@ function drupal_get_filename($type, $name, $filename = NULL) {
  * file.
  */
 function variable_initialize($conf = array()) {
-  // NOTE: caching the variables improves performance by 20% when serving cached pages.
+  // NOTE: caching the variables improves performance by 20% when serving
+  // cached pages.
   if ($cached = cache_get('variables', 'cache_bootstrap')) {
     $variables = $cached->data;
   }
   else {
-    $variables = array_map('unserialize', db_query('SELECT name, value FROM {variable}')->fetchAllKeyed());
-    cache_set('variables', $variables, 'cache_bootstrap');
+    // Cache miss. Avoid a stampede.
+    $name = 'variable_init';
+    if (!lock_acquire($name, 1)) {
+      // Another request is building the variable cache.
+      // Wait, then re-run this function.
+      lock_wait($name);
+      return variable_initialize($conf);
+    }
+    else {
+      // Proceed with variable rebuild.
+      $variables = array_map('unserialize', db_query('SELECT name, value FROM {variable}')->fetchAllKeyed());
+      cache_set('variables', $variables, 'cache_bootstrap');
+      lock_release($name);
+    }
   }
 
   foreach ($conf as $name => $value) {
@@ -2169,6 +2182,10 @@ function _drupal_bootstrap_database() {
 function _drupal_bootstrap_variables() {
   global $conf;
 
+  // Initialize the lock system.
+  require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
+  lock_initialize();
+
   // Load variables from the database, but do not overwrite variables set in settings.php.
   $conf = variable_initialize(isset($conf) ? $conf : array());
   // Load bootstrap modules.
@@ -2182,10 +2199,6 @@ function _drupal_bootstrap_variables() {
 function _drupal_bootstrap_page_header() {
   bootstrap_invoke_all('boot');
 
-  // Prepare for non-cached page workflow.
-  require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
-  lock_initialize();
-
   if (!drupal_is_cli()) {
     ob_start();
     drupal_page_header();