? includes/handlers.inc
? modules/system/system.path_lazy.inc
? modules/system/system.path_precache.inc
Index: includes/bootstrap.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/bootstrap.inc,v
retrieving revision 1.275
diff -u -p -r1.275 bootstrap.inc
--- includes/bootstrap.inc	19 Apr 2009 19:10:07 -0000	1.275
+++ includes/bootstrap.inc	22 Apr 2009 02:48:45 -0000
@@ -1215,6 +1215,8 @@ function _drupal_bootstrap($phase) {
         exit;
       }
       // Prepare for non-cached page workflow.
+      require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
+      lock_init();
       drupal_page_header();
       // If the session has not already been started and output buffering is
       // not enabled, the session must be started now before the HTTP headers
Index: includes/lock.inc
===================================================================
RCS file: includes/lock.inc
diff -N includes/lock.inc
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ includes/lock.inc	22 Apr 2009 02:48:45 -0000
@@ -0,0 +1,181 @@
+<?php
+// $Id$
+
+/**
+ * @file
+ * A database-mediated implementation of a locking mechanism.
+ */
+
+/**
+ * @defgroup locks Lock acquire and release functions
+ * @{
+ * Functions to coordinate long-running operations across requests.
+ *
+ * This is a cooperative, advisory lock system. Any long-running
+ * operation which could potentially be attempted in parallel by multiple
+ * requests should try to acquire a lock before proceeding.  For example,
+ * functions doing a large number of database inserts after building new
+ * data based on a drupal variable's state.  If a function fails to acquire
+ * a lock it should either immediately return, or call lock_wait() if the
+ * rest of the page request requires that the operation in question be
+ * complete.  After lock_wait() returns, the function may again attempt to
+ * acquire a lock, or may simply allow the page request to proceed on the
+ * assumption that a parallel request completed the operation.
+ *
+ * lock_wait() will break stale locks, so that they do not persist.
+ *
+ * A function that has acquired a lock may attempt to renew the lock one
+ * or more times during the operation in order to extend the lifetime of the
+ * lock. Failure to renew a lock is indicative that another request has
+ * acquired the lock, and that the current operation may need to be aborted.
+ *
+ * Alternative implementations of this API (such as APC) my be substituted
+ * by setting the 'lock_inc' variable to an alternate include filepath.
+ */
+
+/**
+ * Initialize the locking system.
+ */
+function lock_init() {
+  global $locks; 
+
+  $locks = array();
+  register_shutdown_function('lock_release_all');
+}
+
+/**
+ * Helper function to get this request's unique id.
+ */
+function _lock_id() {
+  static $lock_id;
+
+  if (!isset($lock_id)) {
+    // Assign a unique id.
+    $lock_id = uniqid(mt_rand());
+  }
+  return $lock_id;
+}
+
+/**
+ * Acquire a lock, but do not block if it fails.
+ *
+ * @param $name
+ *   The name of the lock.
+ * @param $timeout
+ *   A number of seconds (float) before the lock expires.
+ * @return TRUE if the lock was acquired, FALSE if it failed.
+ */
+function lock_acquire($name, $timeout = 20.0) {
+  global $locks;
+
+  if (isset($locks[$name])) {
+    // We acquired a lock before, so check whether our lock is intact.
+    if (db_result(db_query("SELECT 1 FROM {semaphore} WHERE name = '%s' AND value = '%s'", $name, _lock_id()))) {
+      return TRUE;
+    }
+    // The lock was broken.
+    unset($locks[$name]);
+  }
+  elseif (lock_may_be_available($name)) {
+    // Try to acquire the lock.
+    list($usec, $sec) = explode(' ', microtime());
+    $expire = (float)$usec + (float)$sec + $timeout;
+    if (@db_query("INSERT INTO {semaphore} (name, value, expire) VALUES ('%s', '%s', %f)", $name, _lock_id(), $expire)) {
+      $locks[$name] = TRUE;
+      return TRUE;
+    }
+  }
+  return FALSE;
+}
+
+/**
+ * Try to renew (extend) a lock previously acquired.
+ *
+ * @param $name
+ *   The name of the lock.
+ * @param $timeout
+ *   A number of seconds (float) before the lock expires.
+ * @return TRUE if the lock was renewed, FALSE if it failed.
+ */
+function lock_renew($name, $timeout = 20.0) {
+  global $locks;
+
+  if (isset($locks[$name])) {
+    list($usec, $sec) = explode(' ', microtime());
+    $expire = (float)$usec + (float)$sec + $timeout;
+    db_query("UPDATE {semaphore} SET expire = %f WHERE name = '%s' AND value = '%s'", $expire, $name, _lock_id());
+    return (bool)db_affected_rows();
+  }
+  return FALSE;
+}
+
+/**
+ * Check if lock acquired by a different process may be available.
+ *
+ * If an existing lock has expired, it is removed.
+ *
+ * @param $name
+ *   The name of the lock.
+ *
+ * @return TRUE if there is no lock or it was removed, FALSE otherwise.
+ */
+function lock_may_be_available($name) {
+  $lock = db_fetch_array(db_query("SELECT expire, value FROM {semaphore} WHERE name = '%s'", $name));
+  if (!$lock) {
+    return TRUE;
+  }
+  list($usec, $sec) = explode(' ', microtime());
+  $now = (float)$usec + (float)$sec;
+  if ($now > $lock['expire']) {
+    db_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s'", $name, $lock['value']);
+    return (bool)db_affected_rows();
+  }
+  return FALSE;
+}
+
+/**
+ * Wait for a lock to be released.
+ *
+ * @param $name
+ *   The name of the lock.
+ * @param $delay
+ *   The maximum number of seconds to wait.
+ * @return TRUE if the lock holds, FALSE if it is available.
+ */
+function lock_wait($name, $delay = 30) {
+
+  while ($delay--) {
+    sleep(1);
+    if (lock_may_be_available($name)) {
+      return FALSE;
+    }
+  }
+  return TRUE;
+}
+
+/**
+ * Release a lock previously acquired by lock_acquire().
+ *
+ * @param $name
+ *   The name of the lock.
+ */
+function lock_release($name) {
+  global $locks;
+  
+  unset($locks[$name]);
+  db_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s'", $name, _lock_id());
+}
+
+/**
+ * Release all previously acquired locks.
+ */
+function lock_release_all() {
+  global $locks;
+  
+  $locks = array();
+  db_query("DELETE FROM {semaphore} WHERE value = '%s'", _lock_id());
+}
+
+/**
+ * @} End of "defgroup locks".
+ */
Index: includes/menu.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/menu.inc,v
retrieving revision 1.321
diff -u -p -r1.321 menu.inc
--- includes/menu.inc	19 Apr 2009 19:10:07 -0000	1.321
+++ includes/menu.inc	22 Apr 2009 02:48:45 -0000
@@ -1818,16 +1818,28 @@ function menu_cache_clear_all() {
  * is different and leaves stale data in the menu tables.
  */
 function menu_rebuild() {
-  variable_del('menu_rebuild_needed');
+  if (!lock_acquire('menu_rebuild')) {
+    // Wait for another request that is already doing this work.
+    lock_wait('menu_rebuild');
+    return FALSE;
+  }
+
   list($menu, $masks) = menu_router_build();
   _menu_router_save($menu, $masks);
-  _menu_navigation_links_rebuild($menu);
-  menu_cache_clear_all();
-  // Clear the page and block caches.
-  _menu_clear_page_cache();
+  if (lock_renew('menu_rebuild')) {
+    _menu_navigation_links_rebuild($menu);
+    // Clear the menu, page and block caches.
+    menu_cache_clear_all();
+    _menu_clear_page_cache();
+  }
   if (defined('MAINTENANCE_MODE')) {
     variable_set('menu_rebuild_needed', TRUE);
   }
+  else {
+    variable_del('menu_rebuild_needed');
+  }
+  lock_release('menu_rebuild');
+  return TRUE;
 }
 
 /**
Index: modules/system/system.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.install,v
retrieving revision 1.316
diff -u -p -r1.316 system.install
--- modules/system/system.install	20 Apr 2009 02:23:16 -0000	1.316
+++ modules/system/system.install	22 Apr 2009 02:48:45 -0000
@@ -1110,6 +1110,31 @@ function system_schema() {
     'primary key' => array('filename'),
   );
 
+  $schema['semaphore'] = array(
+    'description' => 'Table for storing locks and other flags that should not be cached by the variable system.',
+    'fields' => array(
+      'name' => array(
+        'description' => 'Primary Key: Unique name.',
+        'type' => 'varchar',
+        'length' => 255,
+        'not null' => TRUE,
+        'default' => ''),
+      'value' => array(
+        'description' => 'A value.',
+        'type' => 'varchar',
+        'length' => 255,
+        'not null' => TRUE,
+        'default' => ''),
+      'expire' => array(
+        'description' => 'A Unix timestamp with microseconds indicating when the semaphore should expire.',
+        'type' => 'float',
+        'size' => 'big',
+        'not null' => TRUE),
+      ),
+    'indexes' => array('expire' => array('expire')),
+    'primary key' => array('name'),
+    );
+
   $schema['sessions'] = array(
     'description' => "Drupal's session handlers read and write into the sessions table. Each record represents a user session, either anonymous or authenticated.",
     'fields' => array(
@@ -3244,6 +3269,37 @@ function system_update_7020() {
 }
 
 /**
+ * Add semaphore table.
+ */
+function system_update_7021() {
+  $ret = array();
+
+  $schema['semaphore'] = array(
+    'fields' => array(
+      'name' => array(
+        'type' => 'varchar',
+        'length' => 255,
+        'not null' => TRUE,
+        'default' => ''),
+      'value' => array(
+        'type' => 'varchar',
+        'length' => 255,
+        'not null' => TRUE,
+        'default' => ''),
+      'expire' => array(
+        'type' => 'float',
+        'size' => 'big',
+        'not null' => TRUE),
+      ),
+    'indexes' => array('expire' => array('expire')),
+    'primary key' => array('name'),
+  );
+  db_create_table($ret, 'semaphore', $schema['semaphore']);
+
+  return $ret;
+}
+
+/**
  * @} End of "defgroup updates-6.x-to-7.x"
  * The next series of updates should start at 8000.
  */
Index: modules/locale/locale.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/locale/locale.module,v
retrieving revision 1.238
diff -u -p -r1.238 locale.module
--- modules/locale/locale.module	31 Mar 2009 02:02:21 -0000	1.238
+++ modules/locale/locale.module	22 Apr 2009 02:48:45 -0000
@@ -372,7 +372,7 @@ function locale($string = NULL, $langcod
       if ($cache = cache_get('locale:' . $langcode, 'cache')) {
         $locale_t[$langcode] = $cache->data;
       }
-      else {
+      elseif (lock_acquire('locale_cache_' . $langcode)) {
         // Refresh database stored cache of translations for given language.
         // We only store short strings used in current version, to improve
         // performance and consume less memory.
@@ -381,6 +381,7 @@ function locale($string = NULL, $langcod
           $locale_t[$langcode][$data->source] = (empty($data->translation) ? TRUE : $data->translation);
         }
         cache_set('locale:' . $langcode, $locale_t[$langcode]);
+        lock_release('locale_cache_' . $langcode);
       }
     }
   }