Index: update.php
===================================================================
RCS file: /cvs/drupal/drupal/update.php,v
retrieving revision 1.250
diff -u -p -r1.250 update.php
--- update.php	21 Jan 2008 15:06:41 -0000	1.250
+++ update.php	22 Jan 2008 17:10:35 -0000
@@ -362,6 +362,8 @@ function update_results_page() {
 function update_info_page() {
   // Change query-strings on css/js files to enforce reload for all users.
   _drupal_flush_css_js();
+  // Flush the cache of all data for the update status module.
+  cache_clear_all('*', 'cache_update', '*');
 
   update_task_list('info');
   drupal_set_title('Drupal database update');
Index: modules/update/update.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/update/update.module,v
retrieving revision 1.15
diff -u -p -r1.15 update.module
--- modules/update/update.module	22 Jan 2008 07:56:07 -0000	1.15
+++ modules/update/update.module	22 Jan 2008 17:10:36 -0000
@@ -379,7 +379,7 @@ function update_flush_caches() {
  * Invalidates any cached data relating to update status.
  */
 function update_invalidate_cache() {
-  cache_clear_all('update_info', 'cache_update');
+  cache_clear_all('*', 'cache_update', '*');
 }
 
 /**
Index: modules/update/update.compare.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/update/update.compare.inc,v
retrieving revision 1.5
diff -u -p -r1.5 update.compare.inc
--- modules/update/update.compare.inc	22 Jan 2008 07:45:18 -0000	1.5
+++ modules/update/update.compare.inc	22 Jan 2008 17:10:36 -0000
@@ -23,8 +23,15 @@
 function update_get_projects() {
   static $projects = array();
   if (empty($projects)) {
-    _update_process_info_list($projects, module_rebuild_cache(), 'module');
-    _update_process_info_list($projects, system_theme_data(), 'theme');
+    // Retrieve the projects from cache, if present.
+    $projects = update_project_cache('update_project_projects');
+    if (empty($projects)) {
+      // Still empty, so we have to rebuild the cache.
+      _update_process_info_list($projects, module_rebuild_cache(), 'module');
+      _update_process_info_list($projects, system_theme_data(), 'theme');
+      // Set the projects array into the cache table.
+      cache_set('update_project_projects', $projects, 'cache_update', time() + 3600);  
+    }
   }
   return $projects;
 }
@@ -225,6 +232,13 @@ function update_process_project_info(&$p
  * @see update_process_project_info()
  */
 function update_calculate_project_data($available) {
+  // Retrieve the projects from cache, if present.
+  $projects = update_project_cache('update_project_data');
+  // If $projects is empty, then the cache must be rebuilt.
+  // Otherwise, return the cached data and skip the rest of the function.
+  if (!empty($projects)) {
+    return $projects;
+  }  
   $projects = update_get_projects();
   update_process_project_info($projects);
   foreach ($projects as $project => $project_info) {
@@ -535,5 +549,48 @@ function update_calculate_project_data($
   // contrib module to provide fine-grained settings to ignore specific
   // projects or releases).
   drupal_alter('update_status', $projects);
+
+  // Set the projects array into the cache table.
+  cache_set('update_project_data', $projects, 'cache_update', time() + 3600);
+
+  return $projects;
+}
+
+/**
+ * Retrieve data from {cache_update} or empty the cache when necessary.
+ *
+ * Two very expensive arrays computed by this module are the list of all
+ * installed modules and themes (and .info data, project associations, etc),
+ * and the current status of the site relative to the currently available
+ * releases. These two arrays are cached in the {cache_update} table and used
+ * whenever possible. The cache is cleared whenever the administrator visits
+ * the status report, available updates report, or the module or theme
+ * administration pages, since we should always recompute the most current
+ * values on any of those pages.
+ *
+ * @param $cid
+ *   The cache id of data to return from the cache. Valid options are
+ *   'update_project_data' and 'update_project_projects'.
+ *
+ * @return
+ *   The cached value of the $projects array generated by
+ *   update_calculate_project_data() or update_get_projects(), or an empty
+ *   array when the cache is cleared.
+ */
+function update_project_cache($cid) {
+  $projects = array();
+  // In some cases, we must clear the cache.  Rather than do so on a time
+  // basis, we check for specific paths.
+  $q = $_GET['q'];
+  $paths = array('admin/build/modules', 'admin/build/themes', 'admin/reports', 'admin/reports/updates', 'admin/reports/status', 'admin/reports/updates/check');
+  if (in_array($q, $paths)) {
+    cache_clear_all($cid, 'cache_update');
+  }
+  elseif (variable_get('update_cache_data', TRUE)) {
+    $cache = cache_get($cid, 'cache_update');
+    if (!empty($cache->data)) {
+      $projects = $cache->data;
+    }  
+  }  
   return $projects;
 }