Implement a 404 handler so that if the ESI URL is somehow incorrect, …

…full Drupal 404 pages do not get embedded within another page.

esi.module

@@ -106,7 +106,11 @@ function esi_admin_menu_output_alter(&$admin_menu) {
  */
 function esi_component_load($component) {
   $components = esi_get_components();
-  return array_key_exists($component, $components) ? $components[$component] : FALSE;
+  // Do not return FALSE: even if the requested component is not valid, allow
+  // the menu handler to return control to esi_handle_component() in order to
+  // return a custom ESI 404.  Having an entire theme Drupal 404 page embedded
+  // within another Drupal page would not be desirable.
+  return array_key_exists($component, $components) ? $components[$component] : NULL;
 }
 
 /**

esi.pages.inc

@@ -6,8 +6,16 @@
 
 /**
  * Menu callback to handle an ESI component.
+ *
+ * @see esi_component_load().
  */
 function esi_handle_component($component) {
+  // The menu wildcard loader will return NULL for invalid components, so that
+  // the menu-handler will delegate 404 delivery here.
+  if (empty($component)) {
+    esi_fast_404();
+  }
+
   // Remove the component from the arguments.
   $args = array_slice(func_get_args(),1);
 
@@ -63,3 +71,24 @@ function esi_deliver_esi_component($esi_rendered_component) {
   // activity to commit.
   drupal_page_footer();
 }
+
+/**
+ * Generate a fast 404 (consisting of a single HTML comment).
+ * Full delivery is not desired, because it would typically result in embedding
+ * a complete, themed Drupal 404 page *within* another page.
+ *
+ * @see drupal_fast_404()
+ */
+function esi_fast_404() {
+  drupal_add_http_header('Status', '404 Not Found');
+  drupal_add_http_header('X-ESI', 'Component not recognised');
+  watchdog('page not found', check_plain($_GET['q']), NULL, WATCHDOG_WARNING);
+
+  // Unlike drupal_fast_404(), the HTML furniture (html tag, head tag, etc) are
+  // not delivered 
+  echo "<!-- esi_fast_404 -->\n";
+
+  // Unlike normal page-views, invoking hook_exit() is unneccessary overhead.
+  // This use of exit matches the behaviour of drupal_fast_404().
+  exit;
+}