MDL-65029 questions: change question div id to acutally be unique

This is largely based on Simey Lameze's work.

question/engine/questionattempt.php

@@ -359,6 +359,17 @@ class question_attempt {
         return 'q' . $this->usageid . ':' . $this->slot . '_';
     }
 
+    /**
+     * When the question is rendered, this unique id is added to the
+     * outer div of the question. It can be used to uniquely reference
+     * the question from JavaScript.
+     *
+     * @return string id added to the outer <div class="que ..."> when the question is rendered.
+     */
+    public function get_outer_question_div_unique_id() {
+        return 'question-' . $this->usageid . '-' . $this->slot;
+    }
+
     /**
      * Get one of the steps in this attempt.
      *

question/engine/renderer.php

@@ -89,7 +89,7 @@ class core_question_renderer extends plugin_renderer_base {
 
         $output = '';
         $output .= html_writer::start_tag('div', array(
-            'id' => 'q' . $qa->get_slot(),
+            'id' => $qa->get_outer_question_div_unique_id(),
             'class' => implode(' ', array(
                 'que',
                 $qa->get_question()->qtype->name(),

question/engine/upgrade.txt

@@ -1,5 +1,18 @@
 This files describes API changes for the core question engine.
 
+=== 3.7 ===
+
+1) When a question is rendered, the outer div of the question has an id="q123"
+   added. Unfortunately, this id was not actually unique, leading to bugs like
+   MDL-52572. Therefore, we have had to change it. The id used now is what
+   is returned by the new method $qa->get_outer_question_div_unique_id().
+   The old code that you need to search for and replace with a call to this
+   method is "'q' . $qa->get_slot()"
+
+   Note, the new method has also been added to Moodle 3.5.6 and 3.6.4, but
+   returning the old id. This is to help question types that want to support
+   multiple Moodle versions.
+
 === 3.1, 3.0.3, 2.9.5 ===
 
 1) The field question_display_options::$extrainfocontent is now displayed in the

question/type/ddimageortext/rendererbase.php

@@ -115,7 +115,7 @@ class qtype_ddtoimage_renderer_base extends qtype_with_combined_feedback_rendere
 
         $PAGE->requires->string_for_js('blank', 'qtype_ddimageortext');
         $PAGE->requires->js_call_amd('qtype_ddimageortext/question', 'init',
-                ['q' . $qa->get_slot(), $options->readonly, $question->places]);
+                [$qa->get_outer_question_div_unique_id(), $options->readonly, $question->places]);
 
         if ($qa->get_state() == question_state::$invalid) {
             $output .= html_writer::nonempty_tag('div',

question/type/ddmarker/renderer.php

@@ -97,7 +97,7 @@ class qtype_ddmarker_renderer extends qtype_ddtoimage_renderer_base {
         }
 
         $PAGE->requires->js_call_amd('qtype_ddmarker/question', 'init',
-                ['q' . $qa->get_slot(), $bgimage, $options->readonly, $visibledropzones]);
+                [$qa->get_outer_question_div_unique_id(), $bgimage, $options->readonly, $visibledropzones]);
 
         if ($qa->get_state() == question_state::$invalid) {
             $output .= html_writer::nonempty_tag('div',

question/type/ddwtos/renderer.php

@@ -43,7 +43,7 @@ class qtype_ddwtos_renderer extends qtype_elements_embedded_in_question_text_ren
         $result = parent::formulation_and_controls($qa, $options);
 
         $PAGE->requires->js_call_amd('qtype_ddwtos/ddwtos', 'init',
-                ['q' . $qa->get_slot(), $options->readonly]);
+                [$qa->get_outer_question_div_unique_id(), $options->readonly]);
         return $result;
     }