Use INSERT IGNORE to handle conflicts gracefully.

Author: Cyperghost

wcfsetup/install/files/lib/system/background/BackgroundQueueHandler.class.php

@@ -81,32 +81,21 @@ public function enqueueAt(AbstractBackgroundJob|array $jobs, int $time): void
         $committed = false;
         try {
             WCF::getDB()->beginTransaction();
-            $sql = "INSERT INTO wcf1_background_job
-                                (job, time,identifier)
-                    VALUES      (?, ?, ?)";
+            $sql = "INSERT IGNORE INTO wcf1_background_job
+                                       (job, time,identifier)
+                    VALUES             (?, ?, ?)";
             $statement = WCF::getDB()->prepare($sql);
-            $sql = "SELECT jobID
-                    FROM   wcf1_background_job
-                    WHERE  identifier = ?
-                    FOR UPDATE";
-            $selectJobStatement = WCF::getDB()->prepare($sql);
 
             foreach ($jobs as $job) {
                 $identifier = null;
                 if ($job instanceof AbstractUniqueBackgroundJob) {
-                    // Check if the job is already in the queue
-                    $selectJobStatement->execute([$job->identifier()]);
-                    $jobID = $selectJobStatement->fetchSingleColumn();
-                    if ($jobID !== false) {
-                        continue;
-                    }
                     $identifier = $job->identifier();
                 }
 
                 $statement->execute([
                     \serialize($job),
                     $time,
-                    $identifier
+                    $identifier,
                 ]);
             }
             WCF::getDB()->commitTransaction();