MDL-42598 quiz variant randomisation not random enough.

A student would get the same question variants (for example for a
calculated question) on their first attempt at both copies of a
duplicated quiz.

mod/quiz/locallib.php

@@ -199,7 +199,8 @@ function quiz_start_new_attempt($quizobj, $quba, $attempt, $attemptnumber, $time
     } else {
         $variantoffset = $attemptnumber;
     }
-    $variantstrategy = new question_variant_pseudorandom_no_repeats_strategy($variantoffset);
+    $variantstrategy = new question_variant_pseudorandom_no_repeats_strategy(
+            $variantoffset, $attempt->userid, $quizobj->get_quizid());
 
     if (!empty($forcedvariantsbyslot)) {
         $forcedvariantsbyseed = question_variant_forced_choices_selection_strategy::prepare_forced_choices_array(

question/engine/lib.php

@@ -957,12 +957,15 @@ class question_variant_pseudorandom_no_repeats_strategy
     /** @var int the user id the attempt belongs to. */
     protected $userid;
 
+    /** @var string extra input fed into the pseudo-random code. */
+    protected $extrarandomness = '';
+
     /**
      * Constructor.
      * @param int $attemptno The attempt number.
      * @param int $userid the user the attempt is for (defaults to $USER->id).
      */
-    public function __construct($attemptno, $userid = null) {
+    public function __construct($attemptno, $userid = null, $extrarandomness = '') {
         $this->attemptno = $attemptno;
         if (is_null($userid)) {
             global $USER;
@@ -970,6 +973,10 @@ class question_variant_pseudorandom_no_repeats_strategy
         } else {
             $this->userid = $userid;
         }
+
+        if ($extrarandomness) {
+            $this->extrarandomness = '|' . $extrarandomness;
+        }
     }
 
     public function choose_variant($maxvariants, $seed) {
@@ -977,7 +984,7 @@ class question_variant_pseudorandom_no_repeats_strategy
             return 1;
         }
 
-        $hash = sha1($seed . '|user' . $this->userid);
+        $hash = sha1($seed . '|user' . $this->userid . $this->extrarandomness);
         $randint = hexdec(substr($hash, 17, 7));
 
         return ($randint + $this->attemptno) % $maxvariants + 1;