1 files changed, 115 insertions, 0 deletions
diff --git a/modules/system/system.module b/modules/system/system.module
index 5088b4cf8..e768944e3 100644
--- a/modules/system/system.module
+++ b/modules/system/system.module
@@ -42,6 +42,11 @@ define('DRUPAL_MINIMUM_PGSQL',  '8.3');
 define('DRUPAL_MAXIMUM_TEMP_FILE_AGE', 21600);
 
 /**
+ * Default interval for automatic cron executions in seconds.
+ */
+define('DRUPAL_CRON_DEFAULT_THRESHOLD', 10800);
+
+/**
  * New users will be set to the default time zone at registration.
  */
 define('DRUPAL_USER_TIMEZONE_DEFAULT', 0);
@@ -196,6 +201,9 @@ function system_theme() {
       'arguments' => array('version' => NULL),
     ),
     'system_compact_link' => array(),
+    'system_run_cron_image' => array(
+      'arguments' => array('image_path' => NULL),
+    ),
   ));
 }
 
@@ -496,6 +504,12 @@ function system_menu() {
     '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,
+  );
   $items['admin'] = array(
     'title' => 'Administer',
     'access arguments' => array('access administration pages'),
@@ -2968,3 +2982,104 @@ function system_retrieve_file($url, $destination = NULL, $overwrite = TRUE) {
   
   return $local;
 }
+
+/**
+ * Implement hook_page_alter().
+ */
+function system_page_alter(&$page) {
+  // Automatic cron runs.
+  // @see system_run_cron_image()
+  if (system_run_cron_image_access()) {
+    $page['page_bottom']['run_cron'] = array(
+      // Trigger cron run via AJAX.
+      '#attached_js' => array(
+        '(function($){ $.get(' . drupal_to_js(url('system/run-cron-image')) . '); })(jQuery);' => array('type' => 'inline', 'scope' => 'header'),
+      ),
+      // Trigger cron run for clients not supporting JavaScript (fall-back).
+      '#markup' => theme('system_run_cron_image', 'system/run-cron-image'),
+    );
+  }
+}
+
+/**
+ * Menu callback; executes cron via an image callback.
+ *
+ * This callback runs cron in a separate HTTP request to prevent "mysterious"
+ * slow-downs of regular HTTP requests. It is either invoked via an AJAX request
+ * (if the client's browser supports JavaScript) or by an IMG tag directly in
+ * the page output (for clients not supporting JavaScript). For the latter case,
+ * we need to output a transparent 1x1 image, so the browser does not render the
+ * image's alternate text or a 'missing image placeholder'. The AJAX request
+ * does not process the returned output.
+ *
+ * @see system_page_alter()
+ * @see theme_system_run_cron_image()
+ * @see system_run_cron_image_access()
+ */
+function system_run_cron_image() {
+  drupal_page_is_cacheable(FALSE);
+
+  // Output a transparent 1x1 image to the browser; required for clients not
+  // supporting JavaScript.
+  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);
+    $cron_threshold = variable_get('cron_safe_threshold', DRUPAL_CRON_DEFAULT_THRESHOLD);
+    if (!isset($cron_last) || (REQUEST_TIME - $cron_last > $cron_threshold)) {
+      // Lock cron threshold semaphore.
+      variable_set('cron_threshold_semaphore', REQUEST_TIME);
+      drupal_cron_run();
+      // Release the cron threshold semaphore.
+      variable_del('cron_threshold_semaphore');
+    }
+  }
+
+  exit;
+}
+
+/**
+ * Checks if the feature to automatically run cron is enabled.
+ *
+ * Also used as a menu access callback for this feature.
+ *
+ * @return
+ *   TRUE if cron threshold is enabled, FALSE otherwise.
+ *
+ * @see system_run_cron_image()
+ * @see system_page_alter()
+ */
+function system_run_cron_image_access() {
+  return variable_get('cron_safe_threshold', DRUPAL_CRON_DEFAULT_THRESHOLD) > 0;
+}
+
+/**
+ * Display image used to run cron automatically.
+ *
+ * Renders an image pointing to the automatic cron run menu callback for
+ * graceful degradation when Javascript is not available. The wrapping NOSCRIPT
+ * tag ensures that only browsers not supporting JavaScript render the image.
+ *
+ * @see system_page_alter()
+ * @see system_run_cron_image()
+ * @ingroup themeable
+ */
+function theme_system_run_cron_image($image_path) {
+  return '<noscript><div id="system-cron-image">' . theme('image', $image_path, '', '', array(), FALSE) . '</div></noscript>';
+}
+