Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.408
diff -u -p -r1.408 form.inc
--- includes/form.inc	28 Nov 2009 14:39:31 -0000	1.408
+++ includes/form.inc	29 Nov 2009 19:28:23 -0000
@@ -165,8 +165,6 @@ function drupal_build_form($form_id, &$f
     $form_state['input'] = $form_state['method'] == 'get' ? $_GET : $_POST;
   }
 
-  $cacheable = FALSE;
-
   if (isset($_SESSION['batch_form_state'])) {
     // We've been redirected here after a batch processing : the form has
     // already been processed, so we grab the post-process $form_state value
@@ -182,6 +180,9 @@ function drupal_build_form($form_id, &$f
     // be passed on to the form processing code.
     if (isset($form_state['input']['form_id']) && $form_state['input']['form_id'] == $form_id && !empty($form_state['input']['form_build_id'])) {
       $form = form_get_cache($form_state['input']['form_build_id'], $form_state);
+      // Set $form_build_id to indicate that $form_state may be updated in cache
+      // in case $form_state['cache'] is set.
+      $form_build_id = $form_state['input']['form_build_id'];
     }
 
     // If the previous bit of code didn't result in a populated $form
@@ -210,10 +211,9 @@ function drupal_build_form($form_id, &$f
       }
 
       drupal_prepare_form($form_id, $form, $form_state);
-      // Store a copy of the unprocessed form for caching and indicate that it
-      // is cacheable in case $form_state['cache'] is set.
+      // Store a copy of the unprocessed form to cache in case
+      // $form_state['cache'] is set.
       $original_form = $form;
-      $cacheable = TRUE;
     }
 
     // Now that we know we have a form, we'll process it (validating,
@@ -227,14 +227,16 @@ function drupal_build_form($form_id, &$f
     // have set $form_state['cache'] to indicate that the original form and the
     // $form_state shall be cached. But the form may only be cached if the
     // special 'no_cache' property is not set to TRUE.
-    if (!empty($form_state['cache']) && empty($form_state['no_cache'])) {
-      // Cache the form upon initial build of the form.
-      if ($cacheable) {
+    // $form_build_id will only be set if the form was built from scratch or if
+    // the form was reloaded from cache.
+    if (isset($form_build_id) && !empty($form_state['cache']) && empty($form_state['no_cache'])) {
+      // Cache the original, unprocessed form upon initial build of the form.
+      if (isset($original_form)) {
         form_set_cache($form_build_id, $original_form, $form_state);
       }
       // After processing a cached form, only update the cached form state.
       else {
-        form_set_cache($form_state['input']['form_build_id'], NULL, $form_state);
+        form_set_cache($form_build_id, NULL, $form_state);
       }
     }
   }