<?php

/**
 * @file
 * A module which runs Drupal cron jobs without the cron application.
 */

/**
 * Implements hook_menu().
 */
function poormanscron_menu() {
  $items['poormanscron/run-cron-check'] = array(
    'page callback' => 'poormanscron_run_cron_check',
    'access callback' => 'poormanscron_run_cron_check_access',
    'type' => MENU_CALLBACK,
  );
  return $items;
}

/**
 * Implements hook_page_build().
 */
function poormanscron_page_build(&$page) {
  // Automatic cron runs.
  if (poormanscron_run_cron_check_access()) {
    $page['page_bottom']['poormanscron'] = array(
      // Trigger cron run via AJAX.
      '#attached' => array(
        'js' => array(
          drupal_get_path('module', 'poormanscron') . '/poormanscron.js' => array('weight' => JS_DEFAULT - 5),
          array(
            'data' => array(
              'cron' => array(
                'basePath' => url('poormanscron'),
                'runNext' => variable_get('cron_last', 0) + variable_get('cron_safe_threshold', 10800),
              ),
            ),
            'type' => 'setting',
          ),
        ),
      ),
    );
  }
}

/**
 * Implements hook_robotstxt().
 */
function poormanscron_robotstxt() {
  return array(
    'Disallow: /poormanscron/',
    'Disallow: /?q=poormanscron/',
  );
}

/**
 * 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 poormanscron_run_cron_check()
 */
function poormanscron_run_cron_check_access() {
  return variable_get('cron_safe_threshold', 10800) > 0;
}

/**
 * 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 invoked via an AJAX request
 * (if the client's browser supports JavaScript).
 *
 * @see poormanscron_run_cron_check_access()
 */
function poormanscron_run_cron_check() {
  $cron_run = FALSE;
  $cron_threshold = variable_get('cron_safe_threshold', 10800);

  // 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', 0);
    if (REQUEST_TIME - $cron_last > $cron_threshold) {
      // Lock cron threshold semaphore.
      variable_set('cron_threshold_semaphore', REQUEST_TIME);
      $cron_run = drupal_cron_run();
      // Release the cron threshold semaphore.
      variable_del('cron_threshold_semaphore');

      if ($cron_run) {
        // Truncate the page cache so that cached pages get a new timestamp for
        // the next cron run.
        cache_clear_all('*', 'cache_page', TRUE);
      }
    }
  }

  $cron_last = variable_get('cron_last', 0);
  drupal_add_http_header('Expires', gmdate(DATE_RFC1123, $cron_last + $cron_threshold));

  drupal_json_output(array('cron_run' => $cron_run));
  drupal_page_footer();
}

/**
 * Implements hook_form_FORM_ID_alter().
 */
function poormanscron_form_system_site_information_settings_alter(&$form, $form_state) {
  $form['cron_safe_threshold'] = array(
    '#type' => 'select',
    '#title' => t('Automatically run cron'),
    '#default_value' => variable_get('cron_safe_threshold', 10800),
    '#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'))),
  );
  $form['actions'] += array('#weight' => 100);
  array_unshift($form['#submit'], 'poormanscron_site_information_settings_submit');
}

/**
 * Form submit callback; clears the page cache if cron settings were changed.
 */
function poormanscron_site_information_settings_submit($form, $form_state) {
  if (variable_get('cron_safe_threshold', 10800) != $form_state['values']['cron_safe_threshold']) {
    cache_clear_all('*', 'cache_page', TRUE);
  }
}