Index: includes/bootstrap.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/bootstrap.inc,v
retrieving revision 1.206.2.11
diff -u -p -r1.206.2.11 bootstrap.inc
--- includes/bootstrap.inc	25 Feb 2009 13:49:54 -0000	1.206.2.11
+++ includes/bootstrap.inc	21 Apr 2009 20:13:49 -0000
@@ -1052,6 +1052,8 @@ function _drupal_bootstrap($phase) {
         exit;
       }
       // Prepare for non-cached page workflow.
+      require_once variable_get('lock_inc', './includes/lock.inc');
+      lock_init();
       drupal_page_header();
       break;
 
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	21 Apr 2009 20:13:49 -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 1.255.2.30
diff -u -p -r1.255.2.30 menu.inc
--- includes/menu.inc	30 Mar 2009 12:12:52 -0000	1.255.2.30
+++ includes/menu.inc	21 Apr 2009 20:13:49 -0000
@@ -1669,15 +1669,31 @@ function menu_cache_clear_all() {
  * is different and leaves stale data in the menu tables.
  */
 function menu_rebuild() {
-  variable_del('menu_rebuild_needed');
-  menu_cache_clear_all();
+  if (!lock_acquire('menu_rebuild')) {
+    // Wait for another request that is already doing this work.
+    lock_wait('menu_rebuild');
+    return FALSE;
+  }
+
   $menu = menu_router_build(TRUE);
+  if (lock_renew('menu_rebuild')) {
+    _menu_navigation_links_rebuild($menu);
+    // Clear the page and block caches.
+    menu_cache_clear_all();
+    _menu_clear_page_cache();
+  }
+
   _menu_navigation_links_rebuild($menu);
   // Clear the page and block caches.
   _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 1.238.2.12
diff -u -p -r1.238.2.12 system.install
--- modules/system/system.install	25 Feb 2009 14:02:46 -0000	1.238.2.12
+++ modules/system/system.install	21 Apr 2009 20:13:49 -0000
@@ -956,6 +956,31 @@ function system_schema() {
     'primary key' => array('mlid'),
     );
 
+  $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(
@@ -2548,6 +2573,37 @@ function system_update_6049() {
 }
 
 /**
+ * Add semaphore table.
+ */
+function system_update_6050() {
+  $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-5.x-to-6.x"
  * The next series of updates should start at 7000.
  */