Make the "undo_function" bit optional.

The "undo_function" part of callbacks was required, even though it
might not be required for functionality... Make it optional, so we
don't end up making/using no-op functions to avoid doing anything.

includes/ingest.form.inc

@@ -467,12 +467,14 @@ function islandora_ingest_form_undo_consecutive_callback_steps(array $form, arra
  *   The step which the undo callback is being called on.
  */
 function islandora_ingest_form_undo_callback_step(array $form, array &$form_state, array $step) {
-  $args = array(&$form_state);
-  $args = isset($step['undo_function']['args']) ? array_merge($args, $step['undo_function']['args']) : $args;
-  if (isset($step['undo_function']['file'])) {
-    require_once drupal_get_path('module', $step['module']) . "/" . $step['undo_function']['file'];
+  if (isset($step['undo_function'])) {
+    $args = array(&$form_state);
+    $args = isset($step['undo_function']['args']) ? array_merge($args, $step['undo_function']['args']) : $args;
+    if (isset($step['undo_function']['file'])) {
+      require_once drupal_get_path('module', $step['module']) . "/" . $step['undo_function']['file'];
+    }
+    call_user_func_array($step['undo_function']['function'], $args);
   }
-  call_user_func_array($step['undo_function']['function'], $args);
 }
 
 /**

islandora.api.php

@@ -423,13 +423,13 @@ function hook_islandora_undeletable_datastreams(array $models) {
  *   - form_id: The form building function to call to get the form structure
  *     for this step.
  *   - args: An array of arguments to pass to the form building function.
- *   Required "callback" type specific parameters:
- *   - do_function: An associate array including:
+ *   "Callback" type specific parameters:
+ *   - do_function: A required associative array including:
  *       - 'function': The callback function to be called.
  *       - 'args': An array of arguments to pass to the callback function.
  *       - 'file': A file to include (relative to the module's path, including
  *          the file's extension).
- *   - undo_function: An associate array including:
+ *   - undo_function: An optional associative array including:
  *       - 'function': The callback function to be called to reverse the
  *          executed action in the ingest steps.
  *       - 'args': An array of arguments to pass to the callback function.