MDL-38538 question auto-save back end.

1. Autosave works in some ways just like a normal save. We ultimately
call $behaviour->process_save() to do the work, and create a new step to
hold the data.

2. However, we come in through a completely different route through the
API, starting with separate auto-save methods. This keeps the auto-save
changes mostly separate, and so reduced the chance of breaking existing
working code.

3. When the time comes to store the auto-save step in the database, we
save it using a negative sequence number.

This is a clever trick that not only distinguises these steps, but also
avoids unique key errors when an auto-save and a real action happen
simultaneously. (There are unit tests for these tricky edge cases.)

4. When we load the data back from the database, most of the time the
auto-save steps are loaded back as if they were a real save, and so the
auto-saved data is used when the question is then rendered.

5. However, before we process another action, we remove the auto-saved
step, so it does not appear in the final history.

question/engine/questionusage.php

@@ -511,7 +511,49 @@ class question_usage_by_activity {
      * instead of the data from $_POST.
      */
     public function process_all_actions($timestamp = null, $postdata = null) {
-        // note: we must not use "question_attempt::get_submitted_var()" because there is no attempt instance!!!
+        foreach ($this->get_slots_in_request($postdata) as $slot) {
+            if (!$this->validate_sequence_number($slot, $postdata)) {
+                continue;
+            }
+            $submitteddata = $this->extract_responses($slot, $postdata);
+            $this->process_action($slot, $submitteddata, $timestamp);
+        }
+        $this->update_question_flags($postdata);
+    }
+
+    /**
+     * Process all the question autosave data in the current request.
+     *
+     * If there is a parameter slots included in the post data, then only
+     * those question numbers will be processed, otherwise all questions in this
+     * useage will be.
+     *
+     * This function also does {@link update_question_flags()}.
+     *
+     * @param int $timestamp optional, use this timestamp as 'now'.
+     * @param array $postdata optional, only intended for testing. Use this data
+     * instead of the data from $_POST.
+     */
+    public function process_all_autosaves($timestamp = null, $postdata = null) {
+        foreach ($this->get_slots_in_request($postdata) as $slot) {
+            if (!$this->is_autosave_required($slot, $postdata)) {
+                continue;
+            }
+            $submitteddata = $this->extract_responses($slot, $postdata);
+            $this->process_autosave($slot, $submitteddata, $timestamp);
+        }
+        $this->update_question_flags($postdata);
+    }
+
+    /**
+     * Get the list of slot numbers that should be processed as part of processing
+     * the current request.
+     * @param array $postdata optional, only intended for testing. Use this data
+     * instead of the data from $_POST.
+     * @return array of slot numbers.
+     */
+    protected function get_slots_in_request($postdata = null) {
+        // Note: we must not use "question_attempt::get_submitted_var()" because there is no attempt instance!!!
         if (is_null($postdata)) {
             $slots = optional_param('slots', null, PARAM_SEQUENCE);
         } else if (array_key_exists('slots', $postdata)) {
@@ -526,14 +568,7 @@ class question_usage_by_activity {
         } else {
             $slots = explode(',', $slots);
         }
-        foreach ($slots as $slot) {
-            if (!$this->validate_sequence_number($slot, $postdata)) {
-                continue;
-            }
-            $submitteddata = $this->extract_responses($slot, $postdata);
-            $this->process_action($slot, $submitteddata, $timestamp);
-        }
-        $this->update_question_flags($postdata);
+        return $slots;
     }
 
     /**
@@ -560,6 +595,18 @@ class question_usage_by_activity {
         $this->observer->notify_attempt_modified($qa);
     }
 
+    /**
+     * Process an autosave action on a specific question.
+     * @param int $slot the number used to identify this question within this usage.
+     * @param $submitteddata the submitted data that constitutes the action.
+     */
+    public function process_autosave($slot, $submitteddata, $timestamp = null) {
+        $qa = $this->get_question_attempt($slot);
+        if ($qa->process_autosave($submitteddata, $timestamp)) {
+            $this->observer->notify_attempt_modified($qa);
+        }
+    }
+
     /**
      * Check that the sequence number, that detects weird things like the student
      * clicking back, is OK. If the sequence check variable is not present, returns