Issue #973436 by catch, joseph.olstad, beejeebus, karschsp, pillarsdotnet,...

Issue #973436 by catch, joseph.olstad, beejeebus, karschsp, pillarsdotnet, mcdruid, DamienMcKenna, carlos8f, sun, dsobon, kentr, Damien Tournoud, pounard, Fabianx, xjm, fgm, Steven Jones, David_Rothstein, donquixote, amateescu, MustangGB, Lars Toomre, basicmagic.net, Jeremy, DanPir, nnewton, yonailo, Peter Bowey, RobLoach, gdaw, dsutter, joel_osc, nareshp, izmeez, joelpittet, torgosPizza, crea, tim.plunkett, YesCT, stefan.r, rwohleb: Overzealous locking in variable_initialize()

includes/bootstrap.inc

@@ -1189,19 +1189,21 @@ function variable_initialize($conf = array()) {
     $variables = $cached->data;
   }
   else {
-    // Cache miss. Avoid a stampede.
+    // Cache miss. Avoid a stampede by acquiring a lock. If the lock fails to
+    // acquire, optionally just continue with uncached processing.
     $name = 'variable_init';
-    if (!lock_acquire($name, 1)) {
-      // Another request is building the variable cache.
-      // Wait, then re-run this function.
+    $lock_acquired = lock_acquire($name, 1);
+    if (!$lock_acquired && variable_get('variable_initialize_wait_for_lock', FALSE)) {
       lock_wait($name);
       return variable_initialize($conf);
     }
     else {
-      // Proceed with variable rebuild.
+      // Load the variables from the table.
       $variables = array_map('unserialize', db_query('SELECT name, value FROM {variable}')->fetchAllKeyed());
-      cache_set('variables', $variables, 'cache_bootstrap');
-      lock_release($name);
+      if ($lock_acquired) {
+        cache_set('variables', $variables, 'cache_bootstrap');
+        lock_release($name);
+      }
     }
   }
 

sites/default/default.settings.php

@@ -691,3 +691,13 @@
  * @see user_user_flood_control()
  */
 # $conf['log_user_flood_control'] = FALSE;
+
+/**
+ * Opt out of variable_initialize() locking optimization.
+ *
+ * After lengthy discussion in https://www.drupal.org/node/973436 a change was
+ * made in variable_initialize() in order to avoid excessive waiting under
+ * certain conditions. Set this variable to TRUE in order to opt out of this
+ * optimization and revert to the original behaviour.
+ */
+# $conf['variable_initialize_wait_for_lock'] = FALSE;