Index: includes/module.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/module.inc,v
retrieving revision 1.209
diff -u -p -r1.209 module.inc
--- includes/module.inc	27 Nov 2010 20:41:38 -0000	1.209
+++ includes/module.inc	4 Jan 2011 07:56:58 -0000
@@ -835,16 +835,29 @@ function module_invoke_all() {
  * Array of modules required by core.
  */
 function drupal_required_modules() {
-  $files = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.info$/', 'modules', 'name', 0);
   $required = array();
 
-  // An install profile is required and one must always be loaded.
-  $required[] = drupal_get_profile();
+  // If installation is already finished, all required modules should be enabled
+  // and available through system_get_info().
+  if (variable_get('install_task') == 'done') {
+    foreach (system_get_info('module') as $module => $info) {
+      if (!empty($info['required'])) {
+        $required[] = $module;
+      }
+    }
+  }
+  else {
+    // During site installation, we need to scan the filesystem to list modules.
+    $files = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.info$/', 'modules', 'name', 0);
+
+    // An install profile is required and one must always be loaded.
+    $required[] = drupal_get_profile();
 
-  foreach ($files as $name => $file) {
-    $info = drupal_parse_info_file($file->uri);
-    if (!empty($info) && !empty($info['required']) && $info['required']) {
-      $required[] = $name;
+    foreach ($files as $name => $file) {
+      $info = drupal_parse_info_file($file->uri);
+      if (!empty($info['required'])) {
+        $required[] = $name;
+      }
     }
   }