Integration of Pantheon with the Cache Expiration (expire) module on Drupal

MODULE.module.php

<?php

/**
 * Implements hook_init().
 */
function MODULE_init() {
  // Add Surrogate-Key headers based on path segments.
  // E.g. if the current path is product/some-category/product-name
  // we should end up with the following Surrogate-Keys:
  // product product/some-category product/some-category/product-name
  $path_segments = explode('/', drupal_get_path_alias());

  $surrogate_keys = array();
  $full_url = '';
  foreach ($path_segments as $segment) {
    if (empty($surrogate_keys)) {
      $full_url = $segment;
    }
    else {
      $full_url .= '/' . $segment;
    }
    $surrogate_keys[] = $full_url;
  }
  drupal_add_http_header('Surrogate-Key', implode(' ', $surrogate_keys));
}

/**
 * Implements hook_expire_cache().
 *
 * Provides integration of Pantheon with the Cache Expiration (expire) module.
 *
 * Be sure to uncheck "Include base URL in expires" in Cache Expiration settings.
 */
function MODULE_expire_cache($urls, $wildcards, $object_type, $object) {
  if (variable_get('expire_include_base_url', EXPIRE_INCLUDE_BASE_URL)) {
    return;
  }
  if (!empty($urls) && function_exists('pantheon_clear_edge_paths')) {
    $urls = array_map(function($url) {return "/$url";}, $urls);
    pantheon_clear_edge_paths($urls);
  }
  // For wildcards, we use the Surrogate-Key purging functionality.
  // Surrogate-Key headers are set in the response based on the
  // url path segments.
  // @See MODULE_init().
  $paths = array_keys($wildcards, TRUE);
  if (!empty($paths) && function_exists('pantheon_clear_edge_keys')) {
    pantheon_clear_edge_keys($paths);
  }
}