Index: includes/ajax.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/ajax.inc,v
retrieving revision 1.28
diff -u -p -r1.28 ajax.inc
--- includes/ajax.inc	26 Mar 2010 18:58:12 -0000	1.28
+++ includes/ajax.inc	30 Mar 2010 01:32:08 -0000
@@ -270,7 +270,7 @@ function ajax_form_callback() {
 
   // This call recreates the form relying solely on the $form_state that
   // drupal_process_form() set up.
-  $form = drupal_rebuild_form($form_id, $form_state, $form_build_id);
+  $form = drupal_rebuild_form($form_id, $form_state, $form);
 
   // As part of drupal_process_form(), the element that triggered the form
   // submission is determined, and in the case of AJAX, it might not be a
Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.445
diff -u -p -r1.445 form.inc
--- includes/form.inc	26 Mar 2010 18:58:12 -0000	1.445
+++ includes/form.inc	30 Mar 2010 01:32:09 -0000
@@ -306,14 +306,14 @@ function form_state_defaults() {
  *   may be found in node_forms(), search_forms(), and user_forms().
  * @param $form_state
  *   A keyed array containing the current state of the form.
- * @param $form_build_id
- *   If the AHAH callback calling this function only alters part of the form,
- *   then pass in the existing form_build_id so we can re-cache with the same
- *   csid.
+ * @param $previous_form
+ *   If called from AJAX, where only part of the form will be returned to the
+ *   browser, then pass the previous build of this form, so that the properties
+ *   of the newly built form can be consistent with the prior build.
  * @return
  *   The newly built form.
  */
-function drupal_rebuild_form($form_id, &$form_state, $form_build_id = NULL) {
+function drupal_rebuild_form($form_id, &$form_state, $previous_form = NULL) {
   // AJAX and other contexts may call drupal_rebuild_form() even when
   // $form_state['rebuild'] isn't set, but _form_builder_handle_input_element()
   // needs to distinguish a rebuild from an initial build in order to process
@@ -323,17 +323,24 @@ function drupal_rebuild_form($form_id, &
 
   $form = drupal_retrieve_form($form_id, $form_state);
 
-  if (!isset($form_build_id)) {
-    // We need a new build_id for the new version of the form.
-    $form_build_id = 'form-' . md5(mt_rand());
+  // During AJAX, we need to retain the build id and action of the previous
+  // form build. When the entire form will be resent to the browser, we're not
+  // passed $previous_form, and can generate a new build id and use the action
+  // set by drupal_retrieve_form() or request_uri().
+  if (isset($previous_form)) {
+    $form['#build_id'] = $previous_form['#build_id'];
+    $form['#action'] = $previous_form['#action'];
   }
-  $form['#build_id'] = $form_build_id;
+  else {
+    $form['#build_id'] = 'form-' . md5(mt_rand());
+  }
+
   drupal_prepare_form($form_id, $form, $form_state);
 
   if (empty($form_state['no_cache'])) {
     // We cache the form structure and the form state so it can be retrieved
     // later for validation.
-    form_set_cache($form_build_id, $form, $form_state);
+    form_set_cache($form['#build_id'], $form, $form_state);
   }
 
   // Clear out all group associations as these might be different when
Index: modules/file/file.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/file/file.module,v
retrieving revision 1.23
diff -u -p -r1.23 file.module
--- modules/file/file.module	27 Mar 2010 05:52:49 -0000	1.23
+++ modules/file/file.module	30 Mar 2010 01:32:10 -0000
@@ -239,7 +239,7 @@ function file_ajax_upload() {
 
   // This call recreates the form relying solely on the form_state that the
   // drupal_process_form() set up.
-  $form = drupal_rebuild_form($form_id, $form_state, $form_build_id);
+  $form = drupal_rebuild_form($form_id, $form_state, $form);
 
   // Retrieve the element to be rendered.
   foreach ($form_parents as $parent) {
Index: modules/simpletest/tests/ajax.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/ajax.test,v
retrieving revision 1.7
diff -u -p -r1.7 ajax.test
--- modules/simpletest/tests/ajax.test	26 Mar 2010 18:58:12 -0000	1.7
+++ modules/simpletest/tests/ajax.test	30 Mar 2010 01:32:11 -0000
@@ -204,6 +204,81 @@ class AJAXFormValuesTestCase extends AJA
   }
 }
 
+/**
+ * Tests that form action is not overridden by AJAX submission.
+ */
+class AJAXFormActionTestCase extends AJAXTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'AJAX form action',
+      'description' => 'Tests that form action is not overridden by AJAX submission.',
+      'group' => 'AJAX',
+    );
+  }
+
+  function setUp() {
+    parent::setUp();
+
+    // Create a multi-valued field for 'page' nodes to use for AJAX testing.
+    $field_name = 'field_ajax_test';
+    $field = array(
+      'field_name' => $field_name,
+      'type' => 'text',
+      'cardinality' => FIELD_CARDINALITY_UNLIMITED,
+    );
+    field_create_field($field);
+    $instance = array(
+      'field_name' => $field_name,
+      'entity_type' => 'node',
+      'bundle' => 'page',
+    );
+    field_create_instance($instance);
+
+    // Login a user who can create 'page' nodes.
+    $this->web_user = $this->drupalCreateUser(array('create page content'));
+    $this->drupalLogin($this->web_user);
+  }
+
+  /**
+   * Test an AJAX submission followed by a non-AJAX submission that triggers a validation error.
+   *
+   * Ensure that the form returned after the validation error contains the same
+   * 'action' attribute as the original form, and not the AJAX url.
+   */
+  function testFormAction() {
+    // Get the form for adding a 'page' node. Save the content in a local
+    // variable, because drupalPostAJAX() will replace $this->content.
+    $this->drupalGet('node/add/page');
+    $content = $this->content;
+
+    // Submit an "add another item" AJAX submission and verify it worked by
+    // ensuring it returned two text fields.
+    $commands = $this->discardSettings($this->drupalPostAJAX(NULL, array(), array('field_ajax_test_add_more' => t('Add another item'))));
+    $fragment = simplexml_load_string('<div>' . $commands[0]['data'] . '</div>');
+    $this->assert(count($fragment->xpath('//input[@type="text"]')) == 2, t('AJAX submission succeeded.'));
+
+    // Submit the form with the non-AJAX "Save" button. Leave the title field
+    // blank to trigger a validation error. First restore $this->content,
+    // because drupalPost() needs that to contain the form, not the JSON string
+    // left by drupalPostAJAX().
+    // @todo While not necessary for this test, we would be emulating the
+    //   browser better by calling drupalPost() with the AJAX-modified content
+    //   rather than with the original content from the drupalGet(), but that's
+    //   not possible with the current implementation of drupalPostAJAX(). See
+    //   http://drupal.org/node/384992.
+    $this->drupalSetContent($content);
+    $this->drupalPost(NULL, array(), t('Save'));
+
+    // Ensure that a validation error occurred since this test is for testing
+    // what happens to the form action after a validation error.
+    $this->assertText('Title field is required.', t('Non-AJAX submission correctly triggered a validation error.'));
+
+    // Ensure that the form's action is correct. There might be multiple forms
+    // on the page, so make sure to check the action attribute of the intended
+    // form.
+    $this->assertFieldByXPath('//form[@id="page-node-form" and @action="' . url('node/add/page') . '"]', NULL, t('Re-rendered form contains the correct action value.'));
+  }
+}
 
 /**
  * Miscellaneous AJAX tests using ajax_test module.