Handling possible race condition in exam creation by retry mechanism.

Author: krulis-martin

app/model/repository/GroupExams.php

@@ -5,6 +5,7 @@
 use App\Model\Entity\Group;
 use App\Model\Entity\GroupExam;
 use Doctrine\ORM\EntityManagerInterface;
+use Doctrine\DBAL\Exception\UniqueConstraintViolationException;
 use DateTime;
 use Exception;
 
@@ -38,6 +39,44 @@ public function findPendingForGroup(Group $group): ?GroupExam
         return $exam ? reset($exam) : null;
     }
 
+    /**
+     * Internal helper that tries to find the exam or create it if not present.
+     * It returns null if creation failed due to race condition (expects retry by caller).
+     * @param Group $group
+     * @param DateTime $begin
+     * @param DateTime $end
+     * @param bool $strict
+     * @return GroupExam|null
+     */
+    private function tryFindOrCreate(Group $group, DateTime $begin, DateTime $end, bool $strict): ?GroupExam
+    {
+        $exam = $this->findBy(["group" => $group, "begin" => $begin]);
+        if (count($exam) > 1) {
+            throw new Exception("Data corruption, there is more than one group exam starting at the same time.");
+        }
+
+        if (!$exam) {
+            try {
+                $this->em->getConnection()->executeQuery(
+                    "INSERT INTO group_exams (group_id, begin, end, lock_strict) VALUES (:gid, :begin, :end, :strict)",
+                    [
+                        'gid' => $group->getId(),
+                        'begin' => $begin->format('Y-m-d H:i:s'),
+                        'end' => $end->format('Y-m-d H:i:s'),
+                        'strict' => $strict ? 1 : 0
+                    ]
+                );
+            } catch (UniqueConstraintViolationException) {
+                // race condition, another transaction created the entity meanwhile
+            }
+            return null; // signal caller to retry
+        } else {
+            $exam = reset($exam);
+        }
+
+        return $exam;
+    }
+
     /**
      * Fetch group exam entity by group-begin index. If not present, new entity is created.
      * @param Group $group
@@ -56,18 +95,13 @@ public function findOrCreate(
         $end = $end ?? $group->getExamEnd();
         $strict = $strict === null ? $group->isExamLockStrict() : $strict;
 
-        $exam = $this->findBy(["group" => $group, "begin" => $begin]);
-        if (count($exam) > 1) {
-            throw new Exception("Data corruption, there is more than one group exam starting at the same time.");
+        for ($retries = 0; $retries < 3; $retries++) {
+            $exam = $this->tryFindOrCreate($group, $begin, $end, $strict);
+            if ($exam !== null) {
+                return $exam;
+            }
         }
 
-        if (!$exam) {
-            $exam = new GroupExam($group, $begin, $end, $strict);
-            $this->persist($exam);
-        } else {
-            $exam = reset($exam);
-        }
-
-        return $exam;
+        throw new Exception("Failed to find or create group exam after multiple attempts.");
     }
 }