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/startattempt.php

@@ -209,7 +209,9 @@ if (!($quizobj->get_quiz()->attemptonlast && $lastattempt)) {
         $variantoffset = $attemptnumber;
     }
     $quba->start_all_questions(
-            new question_variant_pseudorandom_no_repeats_strategy($variantoffset), $timenow);
+            new question_variant_pseudorandom_no_repeats_strategy(
+                    $variantoffset, $attempt->userid, $quizobj->get_quizid()),
+            $timenow);
 
     // Update attempt layout.
     $newlayout = array();

question/engine/lib.php

@@ -884,12 +884,15 @@ 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;
@@ -897,6 +900,10 @@ class question_variant_pseudorandom_no_repeats_strategy
         } else {
             $this->userid = $userid;
         }
+
+        if ($extrarandomness) {
+            $this->extrarandomness = '|' . $extrarandomness;
+        }
     }
 
     public function choose_variant($maxvariants, $seed) {
@@ -904,7 +911,7 @@ 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;