=== modified file 'includes/common.inc'
--- includes/common.inc	2009-07-31 07:27:59 +0000
+++ includes/common.inc	2009-07-31 16:32:06 +0000
@@ -2954,8 +2954,12 @@ function drupal_get_js($scope = 'header'
   $no_preprocess = '';
   $files = array();
   $preprocess_js = (variable_get('preprocess_js', FALSE) && (!defined('MAINTENANCE_MODE') || MAINTENANCE_MODE != 'update'));
-  $directory = file_directory_path();
-  $is_writable = is_dir($directory) && is_writable($directory) && (variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC) == FILE_DOWNLOADS_PUBLIC);
+  if ($preprocess_js) {
+  	// Only check the preprocessor requirements if it's going to be
+  	// preprocessing. If these requirements are not met, preprocessing
+  	// will be disabled.
+  	$preprocess_js = drupal_js_preprocessor()->requirements();
+  }
 
   // A dummy query-string is added to filenames, to gain control over
   // browser-caching. The string changes on every update or full cache
@@ -2986,7 +2990,7 @@ function drupal_get_js($scope = 'header'
         break;
 
       case 'file':
-        if (!$item['preprocess'] || !$is_writable || !$preprocess_js) {
+        if (!$item['preprocess'] || !$preprocess_js) {
           $no_preprocess .= '<script type="text/javascript"' . ($item['defer'] ? ' defer="defer"' : '') . ' src="' . base_path() . $item['data'] . ($item['cache'] ? $query_string : '?' . REQUEST_TIME) . "\"></script>\n";
         }
         else {
@@ -3002,12 +3006,8 @@ function drupal_get_js($scope = 'header'
   }
 
   // Aggregate any remaining JS files that haven't already been output.
-  if ($is_writable && $preprocess_js && count($files) > 0) {
-    // Prefix filename to prevent blocking by firewalls which reject files
-    // starting with "ad*".
-    $filename = 'js_' . md5(serialize($files) . $query_string) . '.js';
-    $preprocess_file = drupal_build_js_cache($files, $filename);
-    $preprocessed .= '<script type="text/javascript" src="' . base_path() . $preprocess_file . '"></script>' . "\n";
+  if ($preprocess_js && count($files)) {
+    $preprocessed .= drupal_js_preprocessor()->preprocess($files);
   }
 
   // Keep the order of JS files consistent as some are preprocessed and others are not.
@@ -3265,44 +3265,66 @@ function drupal_add_tabledrag($table_id,
 }
 
 /**
- * Aggregate JS files, putting them in the files directory.
- *
- * @param $files
- *   An array of JS files to aggregate and compress into one file.
- * @param $filename
- *   The name of the aggregate JS file.
- * @return
- *   The name of the JS file.
+ * Returns a JavaScript preprocessing object that implements JSPreprocessingInterface.
  */
-function drupal_build_js_cache($files, $filename) {
-  $contents = '';
-
-  // Create the js/ within the files folder.
-  $jspath = file_create_path('js');
-  file_check_directory($jspath, FILE_CREATE_DIRECTORY);
-
-  if (!file_exists($jspath . '/' . $filename)) {
-    // Build aggregate JS file.
-    foreach ($files as $path => $info) {
-      if ($info['preprocess']) {
-        // Append a ';' after each JS file to prevent them from running together.
-        $contents .= file_get_contents($path) . ';';
+function drupal_js_preprocessor() {
+  static $instance;
+  if (empty($instance)) {
+  	// Retrieve the preprocess system.
+    $class = variable_get('preprocess_js_system', 'DrupalPreprocessJS');
+
+    // Allow for lazy loading of the class.
+    if (drupal_autoload_class($class)) {
+      $interfaces = class_implements($class);
+      if (isset($interfaces['JSPreprocessingInterface'])) {
+        $instance = new $class;
+      }
+      else {
+        throw new Exception(t('Class %class does not implement interface %interface', array('%class' => $class, '%interface' => 'JSPreprocessingInterface')));
       }
     }
-
-    // Create the JS file.
-    file_unmanaged_save_data($contents, $jspath . '/' . $filename, FILE_EXISTS_REPLACE);
+    else {
+      throw new Exception(t('Class %class was not found.', array('%class' => $class)));
+    }
   }
+  return $instance;
+}
+
+/**
+ * The interface for preprocessing JavaScript.
+ */
+interface JSPreprocessingInterface {
+  /**
+   * Check preprocessing requirements.
+   *
+   * @return
+   *   TRUE if requirements met to preform preprocessing.
+   */
+  public function requirements();
+
+  /**
+   * Preprocess JavaScript files.
+   *
+   * @param $files
+   *   An array of JavaScript files.
+   *
+   * @return
+   *   HTML for preprocessed JavaScript files.
+   */
+  public function preprocess($files);
 
-  return $jspath . '/' . $filename;
+  /**
+   * Clear JS cache.
+   */
+  public function clear();
 }
+ 
 
 /**
- * Delete all cached JS files.
+ * Clear JS cache.
  */
 function drupal_clear_js_cache() {
-  file_scan_directory(file_create_path('js'), '/.*/', array('callback' => 'file_unmanaged_delete'));
-  variable_set('javascript_parsed', array());
+  drupal_js_preprocessor()->clear();
 }
 
 /**

=== modified file 'modules/simpletest/tests/common.test'
--- modules/simpletest/tests/common.test	2009-07-30 19:57:09 +0000
+++ modules/simpletest/tests/common.test	2009-07-31 17:03:42 +0000
@@ -504,6 +504,7 @@ class JavaScriptTestCase extends DrupalW
    * Store configured value for JavaScript preprocessing.
    */
   protected $preprocess_js = NULL;
+  protected $preprocess_plugin = NULL;
 
   public static function getInfo() {
     return array(
@@ -520,6 +521,7 @@ class JavaScriptTestCase extends DrupalW
     // Disable preprocessing
     $this->preprocess_js = variable_get('preprocess_js', 0);
     variable_set('preprocess_js', 0);
+    $this->preprocess_plugin = variable_get('preprocess_js_system', 'DrupalPreprocessJS');
 
     // Reset drupal_add_js() and drupal_add_library() statics before each test.
     drupal_static_reset('drupal_add_js');
@@ -529,6 +531,7 @@ class JavaScriptTestCase extends DrupalW
   function tearDown() {
     // Restore configured value for JavaScript preprocessing.
     variable_set('preprocess_js', $this->preprocess_js);
+    variable_set('preprocess_js_system', $this->preprocess_plugin);
     parent::tearDown();
   }
 
@@ -744,6 +747,23 @@ class JavaScriptTestCase extends DrupalW
     $scripts = drupal_get_js();
     $this->assertTrue(strpos($scripts, 'unknown') === FALSE, t('Unknown library was not added to the page.'));
   }
+  
+  /**
+   * Tests the pluggable JavaScript preprocessor system.
+   */
+  function testPluggablePreprocessor() {
+  	// Switch the preprocessor.
+    variable_set('preprocess_js', TRUE);
+  	variable_set('preprocess_js_system', 'testJSPreprocessing');
+
+  	// Add some JavaScript to test the preprocessor system.
+    drupal_add_js('misc/tableselect.js');
+
+    // Retrieve the JavaScript, running through the JavaScriptTestCase preprocessor.
+    $javascript = drupal_get_js();
+    $this->assertTrue(drupal_js_preprocessor()->requirements(), t('Pluggable preprocessor system requirements are checked.'));
+    $this->assertTrue(strpos($javascript, 'has been preprocessed') > 0, t('Preprocessor system is pluggable.'));
+  }
 }
 
 /**

=== modified file 'modules/simpletest/tests/common_test.module'
--- modules/simpletest/tests/common_test.module	2009-07-04 18:26:42 +0000
+++ modules/simpletest/tests/common_test.module	2009-07-31 17:03:29 +0000
@@ -55,3 +55,17 @@ function common_test_library() {
   );
   return $libraries;
 }
+
+class testJSPreprocessing implements JSPreprocessingInterface {
+  public function requirements() {
+    return TRUE;
+  }
+
+  public function preprocess($files) {
+  	return 'JavaScript has been preprocessed.';
+  }
+  
+  public function clear() {
+    
+  }
+}
\ No newline at end of file