Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.421
diff -u -p -r1.421 form.inc
--- includes/form.inc	17 Dec 2009 21:59:31 -0000	1.421
+++ includes/form.inc	28 Dec 2009 08:53:03 -0000
@@ -2,6 +2,18 @@
 // $Id: form.inc,v 1.421 2009/12/17 21:59:31 dries Exp $
 
 /**
+ * Validate the section that is 1 level up from the button.
+ * @see _form_validate_skip()
+ */
+define('VALIDATE_SECTION_PARENT', -1);
+
+/**
+ * Validate the section that is 2 levels up from the button.
+ * @see _form_validate_skip()
+ */
+define('VALIDATE_SECTION_GRANDPARENT', -2);
+
+/**
  * @defgroup forms Form builder functions
  * @{
  * Functions that build an abstract representation of a HTML form.
@@ -897,7 +909,7 @@ function _form_validate(&$elements, &$fo
       // length if it's a string, and the item count if it's an array.
       // An unchecked checkbox has a #value of numeric 0, different than string
       // '0', which could be a valid value.
-      if ($elements['#required'] && (!count($elements['#value']) || (is_string($elements['#value']) && strlen(trim($elements['#value'])) == 0) || $elements['#value'] === 0)) {
+      if ($elements['#required'] && !_form_validate_skip($elements, $form_state) && (!count($elements['#value']) || (is_string($elements['#value']) && strlen(trim($elements['#value'])) == 0) || $elements['#value'] === 0)) {
         form_error($elements, $t('!name field is required.', array('!name' => $elements['#title'])));
       }
 
@@ -935,7 +947,7 @@ function _form_validate(&$elements, &$fo
     }
     // Call any element-specific validators. These must act on the element
     // #value data.
-    elseif (isset($elements['#element_validate'])) {
+    elseif (isset($elements['#element_validate']) && !_form_validate_skip($elements, $form_state)) {
       foreach ($elements['#element_validate'] as $function) {
         if (function_exists($function))  {
           $function($elements, $form_state, $form_state['complete form']);
@@ -947,6 +959,189 @@ function _form_validate(&$elements, &$fo
 }
 
 /**
+ * Check if an element's validation may be skipped.
+ *
+ * A form button may specify the #validate_sections property to limit form
+ * validation to form elements whose values reside within one of the specified
+ * sections of $form_state['values']. Such buttons are required to also define
+ * custom #submit handlers, which ONLY process those sections of
+ * $form_state['values'].
+ *
+ * Example:
+ * @code
+ * function foo_form() {
+ *   $form = array(
+ *     '#tree' => TRUE,
+ *     'title' => array(
+ *       '#type' => 'textfield',
+ *       '#title' => t('Title'),
+ *       '#required' => TRUE,
+ *     ),
+ *     'body' => array(
+ *       '#type' => 'textarea',
+ *       '#title' => t('Body'),
+ *       '#required' => TRUE,
+ *     ),
+ *     'a' => array(
+ *       ...
+ *       'b' => array(
+ *         ...
+ *         'c' => array(
+ *           '#type' => 'textfield',
+ *           '#required' => TRUE,
+ *         ),
+ *       ), 
+ *     ),
+ *     'submit1' => array(
+ *       '#type' => 'submit',
+ *       '#value' => t('Submit 1'),
+ *     ),
+ *     'submit2' => array(
+ *       '#type' => 'submit',
+ *       '#value' => t('Submit 2'),
+ *       '#validate_sections = array(array('body'), array('a', 'b')),
+ *       '#submit' => array('foo_form_partial_submit'),
+ *     ),
+ *   );
+ * }
+ *
+ * function foo_form_submit($form, &$form_state) {
+ *   // This is the form's default submit handler and can assume everything in
+ *   // the form has been validated.
+ * }
+ *
+ * function foo_form_partial_submit($form, &$form_state) {
+ *   // This is the special submit handler for when 'submit2' is clicked. Only
+ *   // the 'body', 'a[b]', and 'a[b][c]' elements have been validated. The
+ *   // 'title' and 'a' elements have not been validated. We can use
+ *   // $form_state['values']['body'] and $form_state['values']['a']['b'],
+ *   // but we must be careful to not rely on any other part of
+ *   // $form_state['values'] containing valid data.
+ * }
+ * @endcode
+ *
+ * This is intended primarily for use in multi-step forms (usually where the
+ * interim steps are submitted with AJAX), such as the "add another item" button
+ * for a multiple value field, where we don't want a not yet filled out but
+ * required field (such as the title) unrelated to the processing of the button
+ * click to cause a validation error, preventing a new item from being added to
+ * the form.
+ *
+ * Each section specified within #validate_sections is an array of successive
+ * keys to reach the desired part of $form_state['values']. This is the same as
+ * the #parents property of the corresponding element to be validated.
+ * Children and nested children of this element also get validated. Note that
+ * because this involves user input validation, #parents rather than
+ * #array_parents is the determining property. Be sure to understand the
+ * difference between these properties when setting #validate_sections.
+ *
+ * In addition to an array that mirrors a #parents array, a section can be set
+ * to a negative number indicating an ancestor of the button being clicked. For
+ * example, setting one of the sections to -3 means to take the button's
+ * #parents property and remove the last 3 entries. It is very common to want to
+ * set the validation section to the parent or grandparent of where the button
+ * is within the form, so these constants are provided:
+ * - VALIDATE_SECTION_PARENT: same as -1
+ * - VALIDATE_SECTION_GRANDPARENT: same as -2
+ * Example:
+ * @code
+ * function foo_form() {
+ *   $form = array(
+ *     '#tree' => TRUE,
+ *     'title' => array(
+ *       ...
+ *     ),
+ *     'group' => array(
+ *       'item1' => array(...),
+ *       'item2' => array(...),
+ *       'group_button' => array(
+ *         '#type' => 'submit',
+ *         '#submit' => array('foo_form_submit_that_only_acts_on_this_group'),
+ *         '#validate_sections' => array(VALIDATE_SECTION_PARENT),
+ *       ),
+ *     ),
+ *     ...
+ *   );
+ * }
+ * @endcode
+ *
+ * To not perform any validation at all, #validate_sections can be set to an
+ * empty array.
+ *
+ * As a final caution, note that using #validate_sections doesn't ensure that
+ * each corresponding part of $form_state['values'] is 100% valid. It only
+ * ensures that the validation handlers for elements AT THAT LEVEL AND DEEPER
+ * have run. Sometimes forms are created with parent elements validating
+ * child elements and these are not run if the parent element is outside of the
+ * specified section. And the form-level validation handlers ARE NOT RUN.
+ * Therefore, be extremely careful when using #validate_sections. It's great for
+ * buttons that are relatively harmless (like an "add another item" button). You
+ * should really know what you're doing if you're using it on a button that has
+ * a submit handler that saves data to the database, sends an email, or does
+ * anything else where input validation is critical.
+ *
+ * @param $element
+ *   The element whose #parents will be checked to determine if this element's
+ *   validation should be skipped.
+ * @param $form_state
+ *   A keyed array containing the current state of the form. The
+ *   'clicked_button' property is checked to determine which elements should be
+ *   validated.
+ *
+ * @return
+ *   Boolean TRUE if the element should not be validated. FALSE if the element
+ *   should be validated.
+ *
+ * @todo D8: This should live in _form_builder_handle_input_element(), because
+ *   ideally, we want to skip not just validation, but all input processing for
+ *   unrelated elements. However, we currently cannot skip input processing,
+ *   because node and field validate/submit handlers cast $form_state['values']
+ *   into an object and presume that doing so results in an object containing
+ *   most properties of an entity.
+ *
+ * @todo Move most of the above documentation to somewhere where developers
+ *   will find it: not buried in a Forms API internal function.
+ */
+function _form_validate_skip($element, $form_state) {
+  // Check whether the pressed button defines #validate_sections to limit
+  // validation to elements within those sections. We prevent insecure execution
+  // of form-level submit handlers by not supporting limited validation unless a
+  // button-level submit handler is defined.
+  if (!(isset($form_state['clicked_button']['#validate_sections']) && isset($form_state['clicked_button']['#submit']))) {
+    return FALSE;
+  }
+  // The element is skipped only if it is not within any of the specified
+  // sections.
+  $skip = TRUE;
+  foreach ($form_state['clicked_button']['#validate_sections'] as $section) {
+    // $section can be a negative number, such as VALIDATE_SECTION_PARENT (-1),
+    // VALIDATE_SECTION_GRANDPARENT (-2), -3, etc. This indicates how far up
+    // the ancestor chain from the button itself to go to find the section that
+    // needs validation. Conveniently, this maps exactly to what array_slice()
+    // does. The way array_slice() works, if $section is a positive number, we
+    // would get back a section that many steps down from the root form. There
+    // might not be a good use-case for setting a positive number, but there's
+    // no reason to disallow it.
+    if (is_numeric($section)) {
+      $section = array_slice($form_state['clicked_button']['#parents'], 0, $section);
+    }
+    // If section is neither numeric (which gets converted to an array above),
+    // nor an array, the developer is not using the API correctly. Perhaps an
+    // error should be triggered.
+    if (is_array($section)) {
+      // If the element is the section itself or a descendant, then it needs
+      // validation.
+      if (count($element['#parents']) >= count($section) && array_slice($element['#parents'], 0, count($section)) === $section) {
+        $skip = FALSE;
+        break;
+      }
+    }
+  }
+
+  return $skip;
+}
+
+/**
  * A helper function used to execute custom validation and submission
  * handlers for a given form. Button-specific handlers are checked
  * first. If none exist, the function falls back to form-level handlers.
@@ -969,7 +1164,13 @@ function form_execute_handlers($type, &$
   }
   // Otherwise, check for a form-level handler.
   elseif (isset($form['#' . $type])) {
-    $handlers = $form['#' . $type];
+    // Check if form-level validation handlers should be skipped.
+    if ($type == 'validate' && _form_validate_skip($form, $form_state)) {
+      $handlers = array();
+    }
+    else {
+      $handlers = $form['#' . $type];
+    }
   }
   else {
     $handlers = array();
Index: modules/field/field.form.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/field/field.form.inc,v
retrieving revision 1.38
diff -u -p -r1.38 field.form.inc
--- modules/field/field.form.inc	21 Dec 2009 13:47:32 -0000	1.38
+++ modules/field/field.form.inc	28 Dec 2009 08:53:04 -0000
@@ -214,7 +214,12 @@ function field_multiple_value_form($fiel
           '#name' => $field_name . '_add_more',
           '#value' => t('Add another item'),
           '#attributes' => array('class' => array('field-add-more-submit')),
-          // Submit callback for disabled JavaScript.
+          // Note that the parent of this button is the language subkey of the
+          // field element. That should be all that needs validation during an
+          // "add more" operation, but if that changes and the entire field
+          // element needs validation, this should be set to
+          // VALIDATE_SECTION_GRANDPARENT.
+          '#validate_sections' => array(VALIDATE_SECTION_PARENT),
           '#submit' => array('field_add_more_submit'),
           '#ajax' => array(
             'callback' => 'field_add_more_js',
Index: modules/poll/poll.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/poll/poll.module,v
retrieving revision 1.330
diff -u -p -r1.330 poll.module
--- modules/poll/poll.module	26 Dec 2009 16:50:09 -0000	1.330
+++ modules/poll/poll.module	28 Dec 2009 08:53:04 -0000
@@ -273,7 +273,13 @@ function poll_form($node, &$form_state) 
     '#value' => t('More choices'),
     '#description' => t("If the amount of boxes above isn't enough, click here to add more choices."),
     '#weight' => 1,
-    '#submit' => array('poll_more_choices_submit'), // If no javascript action.
+    // Normally, #validate_sections is set to array(VALIDATE_SECTION_PARENT),
+    // but that would result in the entire form being validated, because
+    // the container this button is in, 'choice_wrapper', has #tree set to
+    // FALSE, so the parent validation section of the button is the entire form.
+    // All that's really needed is for the 'choice' section to be valildated.
+    '#validate_sections' => array(array('choice')),
+    '#submit' => array('poll_more_choices_submit'),
     '#ajax' => array(
       'callback' => 'poll_choice_js',
       'wrapper' => 'poll-choices',
Index: modules/simpletest/tests/form.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/form.test,v
retrieving revision 1.31
diff -u -p -r1.31 form.test
--- modules/simpletest/tests/form.test	17 Dec 2009 17:18:03 -0000	1.31
+++ modules/simpletest/tests/form.test	28 Dec 2009 08:53:04 -0000
@@ -231,6 +231,46 @@ class FormValidationTestCase extends Dru
     $this->assertNoFieldByName('name', t('Form element was hidden.'));
     $this->assertText('Name value: element_validate_access', t('Value for inaccessible form element exists.'));
   }
+
+  /**
+   * Tests partial form validation using #validate_sections.
+   */
+  function testValidatePartial() {
+    $this->drupalGet('form-test/validate-partial');
+
+    // Add one 'field' value without a value for required 'title' element.
+    $edit = array(
+      'field[new]' => 'foo',
+    );
+    $this->drupalPost(NULL, $edit, 'Add');
+    $this->assertText('Form element validation handler for field triggered.');
+    $this->assertFieldByName('field[0]', $edit['field[new]'], t('First field element contains submitted value.'));
+    $this->assertNoText(t('!name field is required.', array('!name' => 'Title')));
+    $this->assertNoText('Form element validation handler for element triggered.');
+    $this->assertNoText('Form submit handler triggered.');
+
+    // Try to submit the form without posting a title, but also not altering the
+    // default value for 'element' to trigger the element validation handler.
+    $this->drupalPost(NULL, array(), 'Submit');
+    $this->assertText(t('!name field is required.', array('!name' => 'Title')));
+    $this->assertText('Form element validation handler for element triggered.');
+    $this->assertNoText('Form submit handler triggered.');
+
+    // Try again without posting a title.
+    $edit = array(
+      'element' => 'bar',
+    );
+    $this->drupalPost(NULL, $edit, 'Submit');
+    $this->assertText(t('!name field is required.', array('!name' => 'Title')));
+    $this->assertNoText('Form submit handler triggered.');
+
+    // Now do a valid post and verify invocation of submit handler.
+    $edit = array(
+      'title' => 'beer',
+    );
+    $this->drupalPost(NULL, $edit, 'Submit');
+    $this->assertText('Form submit handler triggered.');
+  }
 }
 
 /**
Index: modules/simpletest/tests/form_test.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/form_test.module,v
retrieving revision 1.24
diff -u -p -r1.24 form_test.module
--- modules/simpletest/tests/form_test.module	17 Dec 2009 17:18:03 -0000	1.24
+++ modules/simpletest/tests/form_test.module	28 Dec 2009 08:53:04 -0000
@@ -17,6 +17,12 @@ function form_test_menu() {
     'access arguments' => array('access content'),
     'type' => MENU_CALLBACK,
   );
+  $items['form-test/validate-partial'] = array(
+    'title' => 'Partial form validation using #validate_sections test',
+    'page callback' => 'drupal_get_form',
+    'page arguments' => array('form_test_validate_partial_form'),
+    'access arguments' => array('access content'),
+  );
 
   $items['form_test/tableselect/multiple-true'] = array(
     'title' => 'Tableselect checkboxes test',
@@ -204,6 +210,93 @@ function form_test_validate_form_validat
 }
 
 /**
+ * Form builder for testing partial validation using #validate_sections.
+ */
+function form_test_validate_partial_form($form, &$form_state) {
+  $form['#tree'] = TRUE;
+
+  $form['title'] = array(
+    '#type' => 'textfield',
+    '#title' => 'Title',
+    '#required' => TRUE,
+  );
+  $form['element'] = array(
+    '#type' => 'textfield',
+    '#title' => 'Element',
+    '#default_value' => 'ELEMENT',
+    '#element_validate' => array('form_test_validate_partial_form_element_validate'),
+  );
+
+  $form['field'] = array(
+    '#type' => 'fieldset',
+    '#title' => 'Field',
+  );
+  if (empty($form_state['storage']['field'])) {
+    $form_state['storage']['field'] = array();
+  }
+  // Always add an empty field the user can submit.
+  $form_state['storage']['field']['new'] = '';
+  $weight = 0;
+  foreach ($form_state['storage']['field'] as $delta => $value) {
+    $form['field'][$delta] = array(
+      '#type' => 'textfield',
+      '#title' => 'Field ' . $delta,
+      '#default_value' => $value,
+      '#element_validate' => array('form_test_validate_partial_form_field_validate'),
+      '#weight' => $weight++,
+    );
+  }
+  $form['field']['new']['#weight'] = 999;
+  $form['field']['submit'] = array(
+    '#type' => 'submit',
+    '#value' => 'Add',
+    '#validate_sections' => array(VALIDATE_SECTION_PARENT),
+    '#submit' => array('form_test_validate_partial_form_field_submit'),
+    '#weight' => 1000,
+  );
+
+  $form['submit'] = array('#type' => 'submit', '#value' => 'Submit');
+
+  return $form;
+}
+
+/**
+ * Form element validation handler for 'element' in form_test_validate_partial_form().
+ */
+function form_test_validate_partial_form_element_validate($element, &$form_state) {
+  if (empty($form_state['values']['element']) || $form_state['values']['element'] == $element['#default_value']) {
+    form_error($element, 'Form element validation handler for element triggered.');
+  }
+}
+
+/**
+ * Form element validation handler for 'field' in form_test_validate_partial_form().
+ */
+function form_test_validate_partial_form_field_validate($element, &$form_state) {
+  drupal_set_message('Form element validation handler for field triggered.');
+}
+
+/**
+ * Form element submit handler for form_test_validate_partial_form().
+ */
+function form_test_validate_partial_form_field_submit($form, &$form_state) {
+  // Store the new value and clear out the 'new' field.
+  $form_state['storage']['field'][] = $form_state['values']['field']['new'];
+  unset($form_state['input']['field']['new']);
+  // @todo The form constructor should be able to define this, but currently
+  //   cannot. @see http://drupal.org/node/648170
+  $form_state['rebuild'] = TRUE;
+  $form_state['cache'] = TRUE;
+}
+
+/**
+ * Form submit handler for form_test_validate_partial_form().
+ */
+function form_test_validate_partial_form_submit($form, &$form_state) {
+  drupal_set_message('Form submit handler triggered.');
+}
+
+/**
  * Create a header and options array. Helper function for callbacks.
  */
 function _form_test_tableselect_get_data() {