Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.182
diff -u -r1.182 system.admin.inc
--- modules/system/system.admin.inc	21 Aug 2009 14:28:52 -0000	1.182
+++ modules/system/system.admin.inc	21 Aug 2009 18:22:15 -0000
@@ -1313,6 +1313,14 @@
     '#description' => t('Render all blocks on the default 404 (not found) page. Disabling blocks can help with performance but might leave users with a less functional site.'),
     '#default_value' => variable_get('site_404_blocks', 0)
   );
+  $form['cron_safe_threshold'] = array(
+    '#type' => 'select',
+    '#title' => t('Automatically run cron'),
+    '#default_value' => variable_get('cron_safe_threshold', DRUPAL_DEFAULT_CRON_THRESHOLD),
+    '#options' => array(0 => t('Never')) + drupal_map_assoc(array(3600, 10800, 21600, 43200, 86400, 604800), 'format_interval'),
+    '#description' => t('When enabled, the site will check whether cron has been run in the configured interval and automatically run it upon the next page request. For more information visit the <a href="@status-report-url">status report page</a>.', array('@status-report-url' => url('admin/reports/status'))),
+  );
+
   $form['#validate'][] = 'system_site_information_settings_validate';
 
   return system_settings_form($form);
@@ -1331,6 +1339,11 @@
   if (!menu_valid_path($item)) {
     form_set_error('site_frontpage', t("The path '@path' is either invalid or you do not have access to it.", array('@path' => $item['link_path'])));
   }
+  // Clear cache when enabling or disabling the cron threshold.
+  $cron_threshold = variable_get('cron_safe_threshold', DRUPAL_DEFAULT_CRON_THRESHOLD);
+  if (($cron_threshold > 0 && $form_state['input']['cron_safe_threshold'] == 0) || ($cron_threshold == 0 && $form_state['input']['cron_safe_threshold'] > 0)) {
+    cache_clear_all();
+  }
 }
 
 /**
Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.757
diff -u -r1.757 system.module
--- modules/system/system.module	21 Aug 2009 14:28:52 -0000	1.757
+++ modules/system/system.module	21 Aug 2009 22:19:31 -0000
@@ -42,6 +42,11 @@
 define('DRUPAL_MAXIMUM_TEMP_FILE_AGE', 21600);
 
 /**
+ * Default interval between automatic cron executions in seconds.
+ */
+define('DRUPAL_DEFAULT_CRON_THRESHOLD', 10800);
+
+/**
  * New users will be set to the default time zone at registration.
  */
 define('DRUPAL_USER_TIMEZONE_DEFAULT', 0);
@@ -196,6 +201,9 @@
       'arguments' => array('version' => NULL),
     ),
     'system_compact_link' => array(),
+    'system_run_cron_image' => array(
+      'arguments' => array('image_path' => NULL),
+    ),
   ));
 }
 
@@ -496,6 +504,12 @@
     'access callback' => TRUE,
     'type' => MENU_CALLBACK,
   );
+  $items['system/run-cron-image'] = array(
+    'title' => 'Execute cron',
+    'page callback' => 'system_run_cron_image',
+    'access callback' => 'system_run_cron_image_access',
+    'type' => MENU_CALLBACK,
+  );
   $items['admin'] = array(
     'title' => 'Administer',
     'access arguments' => array('access administration pages'),
@@ -2968,3 +2982,89 @@
   
   return $local;
 }
+
+/**
+ * Implement hook_page_alter().
+ */
+function system_page_alter(&$page) {
+  // We always add triggers to run cron automatically when this feature 
+  // is enabled; it avoids troubles with cached pages.
+  if (system_run_cron_image_access()) {
+    $page['page_bottom']['run_cron'] = array(
+      // GET the image callback with XMLHttpRequest, it will work only
+      // when Javascript is available.
+      '#attached_js' => array(
+        '(function($){ $.get(' . drupal_to_js(url('system/run-cron-image')) . '); })(jQuery);' => array('type' => 'inline', 'scope' => 'header'),
+      ),
+      // Image callback as HTML code for a graceful degradation when Javascript
+      // is not available.
+      '#markup' => theme('system_run_cron_image', 'system/run-cron-image'),
+    );
+  }
+}
+
+/**
+ * Menu callback; executes cron via an image callback.
+ *
+ * We render a transparent 1x1 image whose path will allow creating a valid
+ * HTTP request that will not prevent the original request to finish correctly,
+ * so we do not disturb the user who made the original request. During this new
+ * request, cron will run if it has not run for an amount of time defined in
+ * the administrative settings.
+ */
+function system_run_cron_image() {
+  drupal_page_is_cacheable(FALSE);
+  
+  // Output a transparent 1x1 image to the browser.
+  drupal_set_header('Content-Type', 'image/gif');
+  echo "\x47\x49\x46\x38\x39\x61\x1\x0\x1\x0\x80\xff\x0\xc0\xc0\xc0\x0\x0\x0\x21\xf9\x4\x1\x0\x0\x0\x0\x2c\x0\x0\x0\x0\x1\x0\x1\x0\x0\x2\x2\x44\x1\x0\x3b";
+  
+  // Cron threshold semaphore is used to avoid errors every time the image
+  // callback is displayed when a previous cron is still running.
+  $threshold_semaphore = variable_get('cron_threshold_semaphore', FALSE);
+  if ($threshold_semaphore) {
+    if (REQUEST_TIME - $threshold_semaphore > 3600) {
+      // Either cron has been running for more than an hour or the semaphore
+      // was not reset due to a database error.
+      watchdog('cron', 'Cron has been running for more than an hour and is most likely stuck.', array(), WATCHDOG_ERROR);
+
+      // Release the cron threshold semaphore.
+      variable_del('cron_threshold_semaphore');
+    }
+  }
+  else {
+    // Run cron automatically if it has never run or threshold was crossed.
+    $cron_last = variable_get('cron_last', NULL);
+    $cron_threshold = variable_get('cron_safe_threshold', DRUPAL_DEFAULT_CRON_THRESHOLD);
+    if (!isset($cron_last) || (REQUEST_TIME - $cron_last > $cron_threshold)) {
+      // Lock cron threshold semaphore.
+      variable_set('cron_threshold_semaphore', REQUEST_TIME);
+      drupal_cron_run();
+      // Release the cron threshold semaphore.
+      variable_del('cron_threshold_semaphore');
+    }
+  }
+
+  exit;
+}
+
+/**
+ * Checks if the feature to run cron automatically is enabled.
+ * 
+ * Also used as a menu access callback for this feature.
+ * 
+ * @return
+ *   TRUE if cron threshold is enabled, FALSE otherwise.
+ */
+function system_run_cron_image_access() {
+  return variable_get('cron_safe_threshold', DRUPAL_DEFAULT_CRON_THRESHOLD) > 0;
+}
+
+/**
+ * Display image used to run cron automatically.
+ *
+ * @ingroup themeable
+ */
+function theme_system_run_cron_image($image_path) {
+  return '<noscript><div id="run-cron-image">' . theme('image', $image_path, '', '', array(), FALSE) . '</div></noscript>';
+}
Index: modules/system/system.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.test,v
retrieving revision 1.65
diff -u -r1.65 system.test
--- modules/system/system.test	19 Aug 2009 20:19:37 -0000	1.65
+++ modules/system/system.test	21 Aug 2009 23:22:20 -0000
@@ -391,6 +391,50 @@
   }
 
   /**
+   * Follow every image paths in the previously retrieved content.
+   */
+  function drupalGetAllImages() {
+    foreach ($this->xpath('//img') as $image) {
+      $this->drupalGet($this->getAbsoluteUrl($image['src']));
+    }
+  }
+  
+  /**
+   * Ensure that the cron image callback to run it automatically is working.
+   */
+  function testCronThreshold() {
+    // In the following tests we do not use REQUEST_TIME to track start time,
+    // because we need the exact time when cron is triggered.
+
+    // Ensure cron does not run when the cron threshold is enabled and was
+    // not passed. 
+    $start_cron_last = time();
+    variable_set('cron_last', $start_cron_last);
+    variable_set('cron_safe_threshold', 10);
+    $this->drupalGet('');
+    // Follow every image path on the page.
+    $this->drupalGetAllImages();
+    $this->assertTrue($start_cron_last == variable_get('cron_last', NULL), t('Cron does not run when the cron threshold is not passed.'));
+
+    // Test if cron runs when the cron threshold was passed.
+    $start_cron_last = time() - 15;
+    variable_set('cron_last', $start_cron_last);
+    $this->drupalGet('');
+    // Follow every image path on the page.
+    $this->drupalGetAllImages();
+    $this->assertTrue(variable_get('cron_last', NULL) > $start_cron_last, t('Cron runs when the cron threshold is passed.'));
+
+    // Test if cron does not run when the cron threshold was is disabled.
+    $start_cron_last = time() - 15;
+    variable_set('cron_safe_threshold', 0);
+    variable_set('cron_last', $start_cron_last);
+    $this->drupalGet('');
+    // Follow every image path on the page.
+    $this->drupalGetAllImages();
+    $this->assertTrue($start_cron_last == variable_get('cron_last', NULL), t('Cron does not run when the cron threshold is disabled.'));
+  }
+  
+  /**
    * Ensure that temporary files are removed.
    */
   function testTempFileCleanup() {