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	1 Sep 2010 11:53:33 -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
@@ -5663,14 +5663,23 @@ function drupal_array_set_nested_value(&
  *
  * Instead, use this helper function:
  * @code
- * list($value, $value_exists) = drupal_array_get_nested_value($form, $parents);
+ * $value = drupal_array_get_nested_value($form, $parents);
+ * @endcode
+ *
+ * The return value will be NULL, regardless of whether the actual value is NULL
+ * or whether the requested key does not exist. If if it is required to know
+ * whether the nested array key actually exists, pass a third argument that is
+ * altered by reference:
+ * @code
+ * $value_exists = NULL;
+ * $value = drupal_array_get_nested_value($form, $parents, $value_exists);
  * if ($value_exists) {
- *   // ... do something with $value ...
+ *   // ...
  * }
  * @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
@@ -5679,34 +5688,57 @@ function drupal_array_set_nested_value(&
  *   The array from which to get the value.
  * @param $parents
  *   An array of parent keys of the value, starting with the outermost key.
+ * @param &$key_exists
+ *   (optional) If passed, this function sets this variable to TRUE if each
+ *   parent key specified by $parents exists in the corresponding section of
+ *   $array, and FALSE otherwise. If this function's return value is anything
+ *   other than NULL, then $key_exists is necessarily TRUE, but if this
+ *   function's return value is NULL, then the caller can check the value of
+ *   $key_exists if it needs to have different logic for when a key is missing
+ *   from the array versus when the key exists, but the value is NULL.
  *
  * @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, possibly NULL, if each parent key specified by
+ *   $parents exists in $array. Otherwise, NULL. The $key_exists argument may be
+ *   used to distinguish the two possibilities when NULL is returned.
  *
  * @see drupal_array_set_nested_value()
- * @see drupal_array_value_exists()
  */
-function drupal_array_get_nested_value($array, $parents) {
+function drupal_array_get_nested_value($array, $parents, &$key_exists = NULL) {
+  $result = $array;
   foreach ($parents as $parent) {
-    if (isset($array[$parent])) {
-      $array = $array[$parent];
+    // array_key_exists() is slower than isset() and triggers notices if the
+    // second argument is not an array, so only call it when absolutely
+    // necessary.
+    if (isset($result[$parent]) || (is_array($result) && array_key_exists($parent, $result))) {
+      $result = $result[$parent];
     }
     else {
-      return array(NULL, FALSE);
+      $key_exists = FALSE;
+      return NULL;
     }
   }
-  return array($array, TRUE);
+  $key_exists = 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.
+ * drupal_array_set_nested_value() for details. It is primarily used for form
+ * structures and renderable arrays.
+ *
+ * If it is required to also get the value of the checked nested key, use
+ * drupal_array_get_nested_value() instead.
+ *
+ * If the number of array parent keys is static, this helper function is
+ * unnecessary and the following code can be used instead:
+ * @code
+ * $value_exists = isset($form['signature_settings']['signature']);
+ * $key_exists = array_key_exists('signature', $form['signature_settings']);
+ * @endcode
  *
  * @param $array
  *   The array with the value to check for.
@@ -5716,22 +5748,16 @@ function drupal_array_get_nested_value($
  * @return
  *   TRUE if all the parent keys exist, FALSE otherwise.
  *
- * @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($array, $parents) {
+  // Although this function is similar to PHP's array_key_exists(), its
+  // arguments should be consistent with drupal_array_get_nested_value().
+  $key_exists = NULL;
+  drupal_array_get_nested_value($array, $parents, $key_exists);
+  return $key_exists;
 }
 
-
 /**
  * Provide theme registration for themes across .inc files.
  */
Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.487
diff -u -p -r1.487 form.inc
--- includes/form.inc	30 Aug 2010 17:07:49 -0000	1.487
+++ includes/form.inc	1 Sep 2010 11:39:36 -0000
@@ -959,8 +959,11 @@ 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) {
+      // If the section exists within $form_state['values'], even if the value
+      // is NULL, copy it to $values.
+      $section_exists = NULL;
+      $value = drupal_array_get_nested_value($form_state['values'], $section, $section_exists);
+      if ($section_exists) {
         drupal_array_set_nested_value($values, $section, $value);
       }
     }
@@ -1723,18 +1726,8 @@ function _form_builder_handle_input_elem
     if ($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;
-        }
-      }
+      $input_exists = NULL;
+      $input = drupal_array_get_nested_value($form_state['input'], $element['#parents'], $input_exists);
       // 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 +1799,7 @@ 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'], $element['#parents'])) {
+  if (!drupal_array_nested_key_exists($form_state['values'], $element['#parents'])) {
     form_set_value($element, $element['#value'], $form_state);
   }
 }
Index: modules/file/file.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/file/file.module,v
retrieving revision 1.37
diff -u -p -r1.37 file.module
--- modules/file/file.module	27 Aug 2010 11:54:32 -0000	1.37
+++ modules/file/file.module	1 Sep 2010 11:39:36 -0000
@@ -572,7 +572,7 @@ function file_managed_file_submit($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