Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.1211
diff -u -p -r1.1211 common.inc
--- includes/common.inc	27 Aug 2010 11:54:32 -0000	1.1211
+++ includes/common.inc	27 Aug 2010 16:19:48 -0000
@@ -5649,8 +5649,8 @@ function drupal_array_set_nested_value(&
 /**
  * Retrieves a value from a nested array with variable depth.
  *
- * This helper function should be used when the depth of the array element you
- * are changing may vary (that is, the number of parent keys is variable). It is
+ * This helper function should be used when the depth of the array element being
+ * retrieved may vary (that is, the number of parent keys is variable). It is
  * primarily used for form structures and renderable arrays.
  *
  * Without this helper function the only way to get a nested array value with
@@ -5663,55 +5663,65 @@ function drupal_array_set_nested_value(&
  *
  * Instead, use this helper function:
  * @code
- * list($value, $value_exists) = drupal_array_get_nested_value($form, $parents);
- * if ($value_exists) {
- *   // ... do something with $value ...
- * }
+ * $value = drupal_array_get_nested_value($form, $parents);
  * @endcode
  *
  * However if the number of array parent keys is static, the value should always
- * be get directly rather than calling this function. For instance:
+ * be retrieved directly rather than calling this function. For instance:
  * @code
  * $value = $form['signature_settings']['signature'];
  * @endcode
  *
+ * Note that if this function returns NULL, it could be that either all keys in
+ * $parents exist in $array, and that the value at the innermost key is NULL, or
+ * that not all keys in $parents exist in $array. If you need to know whether
+ * all parent keys exist, you can use the drupal_array_nested_key_exists()
+ * function.
+ *
  * @param $array
  *   The array from which to get the value.
  * @param $parents
  *   An array of parent keys of the value, starting with the outermost key.
  *
  * @return
- *   An indexed array containing:
- *   - The requested nested value, if it exists, or NULL if it does not.
- *   - TRUE if all the parent keys exist, FALSE otherwise.
+ *   The requested nested value, if it exists, or NULL if it does not.
  *
  * @see drupal_array_set_nested_value()
- * @see drupal_array_value_exists()
+ * @see drupal_array_nested_key_exists()
  */
 function drupal_array_get_nested_value($array, $parents) {
+  $result = $array;
   foreach ($parents as $parent) {
-    if (isset($array[$parent])) {
-      $array = $array[$parent];
+    if (isset($result[$parent])) {
+      $result = $result[$parent];
     }
     else {
-      return array(NULL, FALSE);
+      return NULL;
     }
   }
-  return array($array, TRUE);
+  return $result;
 }
 
 /**
- * Determines whether a value in a nested array with variable depth exists.
+ * Determines whether a nested array with variable depth contains all of the requested keys.
  *
  * This helper function should be used when the depth of the array element to be
- * checked may vary (that is, the number of parent keys is variable). See
- * drupal_array_set_nested_value() for details. This helper is primarily used
- * for form structures and renderable arrays.
+ * checked may vary (that is, the number of parent keys is variable). It is
+ * primarily used for form structures and renderable arrays.
+ *
+ * If the number of array parent keys is static, this helper function is
+ * unnecessary and the following code can be used instead:
+ * @code
+ * $key_exists = array_key_exists('signature', $form['signature_settings']);
+ * @endcode
+ *
+ * In PHP, function calls have overhead, so within a performance-critical
+ * function, the contents of this function should be inlined.
  *
- * @param $array
- *   The array with the value to check for.
  * @param $parents
  *   An array of parent keys of the value, starting with the outermost key.
+ * @param $array
+ *   The array with the value to check for.
  *
  * @return
  *   TRUE if all the parent keys exist, FALSE otherwise.
@@ -5719,19 +5729,12 @@ function drupal_array_get_nested_value($
  * @see drupal_array_set_nested_value()
  * @see drupal_array_get_nested_value()
  */
-function drupal_array_nested_value_exists($array, $parents) {
-  foreach ($parents as $parent) {
-    if (isset($array[$parent])) {
-      $array = $array[$parent];
-    }
-    else {
-      return FALSE;
-    }
-  }
-  return TRUE;
+function drupal_array_nested_key_exists($parents, $array) {
+  $last_parent = array_pop($parents);
+  $parent_array = drupal_array_get_nested_value($array, $parents);
+  return isset($parent_array) && is_array($parent_array) && array_key_exists($last_parent, $parent_array);
 }
 
-
 /**
  * Provide theme registration for themes across .inc files.
  */
Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.485
diff -u -p -r1.485 form.inc
--- includes/form.inc	27 Aug 2010 11:54:32 -0000	1.485
+++ includes/form.inc	27 Aug 2010 16:19:48 -0000
@@ -959,9 +959,13 @@ function drupal_validate_form($form_id, 
   if (isset($form_state['triggering_element']['#limit_validation_errors']) && $form_state['triggering_element']['#limit_validation_errors'] !== FALSE) {
     $values = array();
     foreach ($form_state['triggering_element']['#limit_validation_errors'] as $section) {
-      list($value, $value_exists) = drupal_array_get_nested_value($form_state['values'], $section);
-      if ($value_exists) {
-        drupal_array_set_nested_value($values, $section, $value);
+      // If the section exists within $form_state['values'], even if the value
+      // is NULL, copy it to $values.
+      $parent_section = $section;
+      $last_section_key = array_pop($parent_section);
+      $parent_value = drupal_array_get_nested_value($form_state['values'], $parent_section);
+      if (isset($parent_value) && is_array($parent_value) && array_key_exists($last_section_key, $parent_value)) {
+        drupal_array_set_nested_value($values, $section, $parent_value[$last_section_key]);
       }
     }
     // For convenience we always make the value of the pressed button available.
@@ -1723,18 +1727,12 @@ function _form_builder_handle_input_elem
     if ($process_input) {
       // Get the input for the current element. NULL values in the input need to
       // be explicitly distinguished from missing input. (see below)
-      $input = $form_state['input'];
-      $input_exists = TRUE;
-      foreach ($element['#parents'] as $parent) {
-        if (is_array($input) && array_key_exists($parent, $input)) {
-          $input = $input[$parent];
-        }
-        else {
-          $input = NULL;
-          $input_exists = FALSE;
-          break;
-        }
-      }
+      $parents = $element['#parents'];
+      $last_parent = array_pop($parents);
+      $input_parent = drupal_array_get_nested_value($form_state['input'], $parents);
+      $input_exists = isset($input_parent) && is_array($input_parent) && array_key_exists($last_parent, $input_parent);
+      $input = $input_exists ? $input_parent[$last_parent] : NULL;
+
       // For browser-submitted forms, the submitted values do not contain values
       // for certain elements (empty multiple select, unchecked checkbox).
       // During initial form processing, we add explicit NULL values for such
@@ -1806,7 +1804,7 @@ function _form_builder_handle_input_elem
 
   // Set the element's value in $form_state['values'], but only, if its key
   // does not exist yet (a #value_callback may have already populated it).
-  if (!drupal_array_nested_value_exists($form_state['values'], $element['#parents'])) {
+  if (!drupal_array_nested_key_exists($element['#parents'], $form_state['values'])) {
     form_set_value($element, $element['#value'], $form_state);
   }
 }
Index: modules/file/file.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/file/file.module,v
retrieving revision 1.37
diff -u -p -r1.37 file.module
--- modules/file/file.module	27 Aug 2010 11:54:32 -0000	1.37
+++ modules/file/file.module	27 Aug 2010 16:19:50 -0000
@@ -572,7 +572,7 @@ function file_managed_file_submit($form,
   // and set $element to the managed_file element that contains that button.
   $parents = $form_state['triggering_element']['#array_parents'];
   $button_key = array_pop($parents);
-  list($element) = drupal_array_get_nested_value($form, $parents);
+  $element = drupal_array_get_nested_value($form, $parents);
 
   // No action is needed here for the upload button, because all file uploads on
   // the form are processed by file_managed_file_value() regardless of which