Built-in cache warmer

We could use something like:

  //Cache warming.
  // Return all nids of nodes not of type 'php'.
  $nids = db_select('node', 'n')
    ->fields('n', array('nid'))
    ->fields('n', array('type'))
    ->condition('n.type', 'php', '<>')
    ->execute()
    ->fetchCol(); // returns an indexed array
  // Now visit each node.
 global $base_url;
 foreach ($nids as $nid) {
    drupal_http_request($base_url . '/node/' . $nid);
  }

A test showed it works as expected. Maybe to extend with:

• configuration to limit it to certain content types or access rules
• run on cron (configurable as the block cache expiration)
• screen and watchdog message on time-out and on which nodes it occurs
• fallback to cURL if drupal_http_request() returns errors
• adding a checkbox add the bottom of the config form to rebuild the cache of all content (can take a long time).

To take care of:
Make sure that if cron is not set it doesn't have the opposite effect, as in D7 cron runs then on first visitor after expiration.

TODO:

• screen and watchdog message on time-out and on which nodes it occurs
• fallback to cURL if drupal_http_request() returns errors
• handle cache warming in batch to avoid time-out
• Detect if automatic running of Cron is disabled over at Administration » Configuration » System » Cron. If so give a recommendation to do so and why. See also http://colans.net/blog/improving-drupal-7-performance-after-period-inact....
• test to run cron without cache warmer kicking in (interval not expired yet). If pages load slow we should recommend to change the Minimum cache lifetime at admin/config/development/performance to be the same as the Flickr Cache Warmer settings or maybe even use its value instead.

NOTE: The Flickr Cache Warmer only seems to work well if you set the 'Minimum cache lifetime' to 'none'.