Index: index.php
===================================================================
RCS file: /cvs/drupal/drupal/index.php,v
retrieving revision 1.94
diff -u -p -r1.94 index.php
--- index.php	26 Dec 2007 08:46:48 -0000	1.94
+++ index.php	8 May 2008 12:57:46 -0000
@@ -14,26 +14,5 @@
 
 require_once './includes/bootstrap.inc';
 drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
-
-$return = menu_execute_active_handler();
-
-// Menu status constants are integers; page content is a string.
-if (is_int($return)) {
-  switch ($return) {
-    case MENU_NOT_FOUND:
-      drupal_not_found();
-      break;
-    case MENU_ACCESS_DENIED:
-      drupal_access_denied();
-      break;
-    case MENU_SITE_OFFLINE:
-      drupal_site_offline();
-      break;
-  }
-}
-elseif (isset($return)) {
-  // Print any value (including an empty string) except NULL or undefined:
-  print theme('page', $return);
-}
-
+drupal_page_content();
 drupal_page_footer();
Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.765
diff -u -p -r1.765 common.inc
--- includes/common.inc	6 May 2008 12:18:45 -0000	1.765
+++ includes/common.inc	8 May 2008 12:57:52 -0000
@@ -271,8 +271,8 @@ function drupal_get_destination() {
  * Drupal will ensure that messages set by drupal_set_message() and other
  * session data are written to the database before the user is redirected.
  *
- * This function ends the request; use it rather than a print theme('page')
- * statement in your menu callback.
+ * This function ends the request; use it rather than a
+ * return array('page', ...) statement in your menu callback.
  *
  * @param $path
  *   A Drupal path or a full URL.
@@ -362,7 +362,7 @@ function drupal_not_found() {
   }
 
   // To conserve CPU and bandwidth, omit the blocks.
-  print theme('page', $return, FALSE);
+  return array('page', $return, FALSE);
 }
 
 /**
@@ -389,7 +389,7 @@ function drupal_access_denied() {
     drupal_set_title(t('Access denied'));
     $return = t('You are not authorized to access this page.');
   }
-  print theme('page', $return);
+  return array('page', $return);
 }
 
 /**
@@ -1454,6 +1454,85 @@ function l($text, $path, $options = arra
 }
 
 /**
+ * Return an array representing the rendered page.
+ *
+ * @param $content
+ *   Follows the rules of the return value of the active menu handler.
+ *
+ * @return
+ *   An array of page elements.
+ *     - 'success' is TRUE or FALSE depending on the success of the request
+ *     - 'data' is the error constant value on error, a variable array otherwise 
+ */
+function drupal_page($content) {
+  $return = array('success' => TRUE);
+  switch ($content) {
+    case MENU_NOT_FOUND:
+    case MENU_ACCESS_DENIED:
+    case MENU_SITE_OFFLINE:
+      $return['success'] = FALSE;
+      $return['data'] = $content;
+      break;
+    default:
+      init_theme();
+      $hooks = theme_get_registry();
+      if (!isset($hooks['page'])) {
+        $return['success'] = FALSE;
+        break;
+      }
+      $info = $hooks['page'];
+      $variables = array('content' => $content);
+      if (isset($info['preprocess functions']) && is_array($info['preprocess functions'])) {
+        // We keep a reference through call_user_func_array().
+        $args = array(&$variables, $hook);
+        foreach ($info['preprocess functions'] as $preprocess_function) {
+          if (function_exists($preprocess_function)) {
+            call_user_func_array($preprocess_function, $args);
+          }
+        }
+      }
+      $return['data'] = $variables;
+      break;
+  }
+  return $return;
+}
+
+/**
+ * Call the active menu handler and output the result.
+ */
+function drupal_page_content() {
+  $return = menu_execute_active_handler();
+
+  if (isset($return)) {
+    // Menu status constants are integers; page content is a string, if defined.
+    if (is_int($return)) {
+      switch ($return) {
+        case MENU_NOT_FOUND:
+          list($hook, $content, $blocks) = drupal_not_found();
+          break;
+        case MENU_ACCESS_DENIED:
+          list($hook, $content, $blocks) = drupal_access_denied();
+          break;
+        case MENU_SITE_OFFLINE:
+          list($hook, $content, $blocks) = drupal_site_offline();
+          break;
+      }
+    }
+    else {
+      list($hook, $content, $blocks) = array('page', $return, TRUE);
+      $return = TRUE;
+    }
+  }
+  
+  if (isset($hook)) {
+    // $hook should be a 'page' type hook, like 'maintenance_page'.
+    print theme($hook, $content, $blocks, TRUE, $return);
+  }
+  
+  // No output presented otherwise.
+}
+
+/**
  * Perform end-of-request tasks.
  *
  * This function sets the page cache if appropriate, and allows modules to
Index: includes/theme.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/theme.inc,v
retrieving revision 1.422
diff -u -p -r1.422 theme.inc
--- includes/theme.inc	6 May 2008 12:18:45 -0000	1.422
+++ includes/theme.inc	8 May 2008 12:57:52 -0000
@@ -589,6 +589,9 @@ function theme() {
     include_once($include_file);
   }
   if (isset($info['function'])) {
+    // Enable the redirection of output into formats other than XHTML,
+    // e.g. XML, JSON.
+    drupal_set_renderer($info['function'], $hook, $args);
     // The theme call is a function.
     $output = call_user_func_array($info['function'], $args);
   }
@@ -659,6 +662,9 @@ function theme() {
         $template_file = $info['path'] . '/' . $template_file;
       }
     }
+    // Enable the redirection of output into formats other than XHTML,
+    // e.g. XML, JSON.
+    drupal_set_renderer($render_function, $hook, $variables);
     $output = $render_function($template_file, $variables);
   }
   // restore path_to_theme()
@@ -1922,3 +1928,36 @@ function template_preprocess_block(&$var
   $variables['template_files'][] = 'block-' . $variables['block']->module . '-' . $variables['block']->delta;
 }
 
+/**
+ * Enable modules to override the standard theme renderer for a theme call.
+ *
+ * Modules invoking hook_renderer() are called in order of weight,
+ * starting with the highest, and only the first response is accepted.
+ */
+function drupal_set_renderer(&$render_function, $hook, &$variables) {
+  foreach (array_reverse(module_implements('renderer')) as $module) {
+    // Construction needed to pass by reference.
+    $args = array($hook, &$variables);
+    if ($function = call_user_func_array($module . '_renderer', $args)) {
+      $render_function = $function;
+      return;
+    }
+  }
+}
+
+/**
+ * Render function to output themed data in JSON format.
+ *
+ * The theme system passes two arguments, a template file path and the
+ * content to be rendered. Here we need only the second.
+ */
+function drupal_render_js() {
+  $args = func_get_args();
+  print drupal_to_js(
+    array(
+      'success' => TRUE,
+      'content' => array_pop($args),
+    )
+  );
+  exit();
+}
Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.74
diff -u -p -r1.74 system.admin.inc
--- modules/system/system.admin.inc	5 May 2008 21:10:48 -0000	1.74
+++ modules/system/system.admin.inc	8 May 2008 12:57:53 -0000
@@ -1866,7 +1866,7 @@ function system_batch_page() {
   elseif (isset($output)) {
     // Force a page without blocks or messages to
     // display a list of collected messages later.
-    print theme('page', $output, FALSE, FALSE);
+    return array('page', $output, FALSE, FALSE);
   }
 }
 
Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.601
diff -u -p -r1.601 system.module
--- modules/system/system.module	6 May 2008 12:18:50 -0000	1.601
+++ modules/system/system.module	8 May 2008 12:57:56 -0000
@@ -661,6 +661,34 @@ function _system_themes_access($theme) {
 }
 
 /**
+ * Implementation of hook_renderer().
+ *
+ * If a page or item has been requested via jQuery, return it in JSON format.
+ */
+function system_renderer($hook, &$variables) {
+  global $theme_key;
+
+  // jQuery sets a HTTP_X_REQUESTED_WITH header of 'XMLHttpRequest'.
+  if (isset($_SERVER['HTTP_X_REQUESTED_WITH']) && $_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest') {
+    // If this is a full page request that has reached the rendering stage,
+    // return it in JSON.
+    if ($hook == 'page') {
+      // Strip out all but a subset of page properties.
+      // This prevents security issues involved with passing unthemed
+      // data to the client and minimizes transfer bandwidth.
+      $use_variables = array('content', 'tabs', 'title', 'show_blocks', 'show_messages', 'menu_result', 'sidebar_left', 'sidebar_right', 'help', 'messages', 'mission', 'primary_links', 'secondary_links');
+      $use_variables = array_merge($use_variables, array_keys(system_region_list($theme_key)));
+      foreach (array_keys($variables) as $key) {
+        if (!in_array($key, $use_variables)) {
+          unset($variables[$key]);
+        }
+      }
+      return 'drupal_render_js';
+    }
+  }
+}
+
+/**
  * Implementation of hook_init().
  */
 function system_init() {