Proposed solution 2. Just rely on the CAS cache.

question.php

@@ -117,6 +117,16 @@ class qtype_stack_question extends question_graded_automatically_with_countback
      */
     protected $questionnoteinstantiated;
 
+    /**
+     * @var array stack_cas_session STACK specific: session of variables.
+     */
+    public $questiontextinstantiated;
+
+    /**
+     * @var array stack_cas_session STACK specific: session of variables.
+     */
+    public $specificfeedbackinstantiated;
+
     /**
      * The next three fields cache the results of some expensive computations.
      * The chache is only vaid for a particular response, so we store the current
@@ -225,6 +235,13 @@ class qtype_stack_question extends question_graded_automatically_with_countback
         }
         $step->set_qt_var('_seed', $this->seed);
 
+        $this->initialise_question_from_seed();
+    }
+
+    /**
+     * Once we know the random seed, we can initialise all the other parts of the question.
+     */
+    public function initialise_question_from_seed() {
         // Build up the question session out of all the bits that need to go into it.
         // 1. question variables.
         $questionvars = new stack_cas_keyval($this->questionvariables, $this->options, $this->seed, 't');
@@ -272,14 +289,10 @@ class qtype_stack_question extends question_graded_automatically_with_countback
                     $session->get_errors($this->user_can_edit()));
         }
 
-        // Now store the values that depend on the instantiated session.
-        $step->set_qt_var('_questionvars', $session->get_keyval_representation());
-        $step->set_qt_var('_questiontext', $questiontext->get_display_castext());
-        $step->set_qt_var('_feedback', $feedbacktext->get_display_castext());
-        $this->questionnoteinstantiated = $notetext->get_display_castext();
-        $step->set_qt_var('_questionnote', $this->questionnoteinstantiated);
-
         // Finally, store only those values really needed for later.
+        $this->questiontextinstantiated     = $questiontext->get_display_castext();
+        $this->specificfeedbackinstantiated = $feedbacktext->get_display_castext();
+        $this->questionnoteinstantiated     = $notetext->get_display_castext();
         $session->prune_session($session_length);
         $this->session = $session;
 
@@ -289,10 +302,7 @@ class qtype_stack_question extends question_graded_automatically_with_countback
 
     public function apply_attempt_state(question_attempt_step $step) {
         $this->seed = (int) $step->get_qt_var('_seed');
-        $questionvars = new stack_cas_keyval($step->get_qt_var('_questionvars'), $this->options, $this->seed, 't');
-        $this->session = $questionvars->get_session();
-        $this->questionnoteinstantiated = $step->get_qt_var('_questionnote');
-        $this->adapt_inputs();
+        $this->initialise_question_from_seed();
     }
 
     /**

renderer.php

@@ -37,7 +37,7 @@ class qtype_stack_renderer extends qtype_renderer {
 
         $response = $qa->get_last_qt_data();
 
-        $questiontext = $qa->get_last_qt_var('_questiontext');
+        $questiontext = $question->questiontextinstantiated;
         $questiontext = $question->format_text(
                 stack_maths::process_display_castext($questiontext),
                 $question->questiontextformat,
@@ -164,7 +164,7 @@ class qtype_stack_renderer extends qtype_renderer {
     protected function stack_specific_feedback_errors_only(question_attempt $qa) {
         $question = $qa->get_question();
         $response = $qa->get_last_qt_data();
-        $feedbacktext = $qa->get_last_qt_var('_feedback');
+        $feedbacktext = $question->specificfeedbackinstantiated;
         if (!$feedbacktext) {
             return '';
         }
@@ -204,7 +204,7 @@ class qtype_stack_renderer extends qtype_renderer {
 
         $question = $qa->get_question();
         $response = $qa->get_last_qt_data();
-        $feedbacktext = $qa->get_last_qt_var('_feedback');
+        $feedbacktext = $question->specificfeedbackinstantiated;
         if (!$feedbacktext) {
             return '';
         }

replacedollars.php

@@ -50,7 +50,6 @@ $questionfields = array('questiontext', 'generalfeedback');
 $qtypestackfields = array('specificfeedback', 'prtcorrect', 'prtpartiallycorrect', 'prtincorrect', 'questionnote');
 $prtnodefields = array('truefeedback', 'falsefeedback');
 $qafields = array('questionsummary', 'rightanswer', 'responsesummary');
-$stepdatanames = "'_questiontext', '_feedback', '_questionnote'";
 $anychanges = false;
 
 // Display.
@@ -128,23 +127,6 @@ foreach ($categories as $key => $category) {
             }
             $anychanges = $anychanges || $changes;
         }
-
-        // Fields in the question_attempt_step_data table.
-        $attemptdata = $DB->get_records_sql("
-                    SELECT qasd.*
-                      FROM {question_attempt_step_data} qasd
-                      JOIN {question_attempt_steps} qas ON qasd.attemptstepid = qas.id
-                      JOIN {question_attempts} qa ON qa.id = qas.questionattemptid
-                     WHERE qa.questionid = :questionid
-                       AND qasd.name IN ({$stepdatanames})
-                ", array('questionid' => $question->id));
-        foreach ($attemptdata as $data) {
-            $changes = $fixer->fix_question_field($data, 'value');
-            if ($changes && $confirm) {
-                $DB->update_record('question_attempt_step_data', $data);
-            }
-            $anychanges = $anychanges || $changes;
-        }
     }
 }