fix(cron): Fix infinite loop on ParallelAware blocked jobs

Signed-off-by: Joas Schilling <coding@schilljs.com>

lib/private/BackgroundJob/JobList.php

@@ -24,6 +24,9 @@ use function min;
 use function strlen;
 
 class JobList implements IJobList {
+	/** @var array<string, int> */
+	protected array $alreadyVisitedParallelBlocked = [];
+
 	public function __construct(
 		protected IDBConnection $connection,
 		protected IConfig $config,
@@ -198,6 +201,12 @@ class JobList implements IJobList {
 			$job = $this->buildJob($row);
 
 			if ($job instanceof IParallelAwareJob && !$job->getAllowParallelRuns() && $this->hasReservedJob(get_class($job))) {
+				if (!isset($this->alreadyVisitedParallelBlocked[get_class($job)])) {
+					$this->alreadyVisitedParallelBlocked[get_class($job)] = $job->getId();
+				} elseif ($this->alreadyVisitedParallelBlocked[get_class($job)] === $job->getId()) {
+					$this->logger->info('Skipped through all jobs and revisited a IParallelAwareJob blocked job again, giving up.', ['app' => 'cron']);
+					return null;
+				}
 				$this->logger->info('Skipping ' . get_class($job) . ' job with ID ' . $job->getId() . ' because another job with the same class is already running', ['app' => 'cron']);
 
 				$update = $this->connection->getQueryBuilder();
@@ -210,6 +219,10 @@ class JobList implements IJobList {
 				return $this->getNext($onlyTimeSensitive, $jobClasses);
 			}
 
+			if ($job !== null && isset($this->alreadyVisitedParallelBlocked[get_class($job)])) {
+				unset($this->alreadyVisitedParallelBlocked[get_class($job)]);
+			}
+
 			if ($job instanceof \OCP\BackgroundJob\TimedJob) {
 				$now = $this->timeFactory->getTime();
 				$nextPossibleRun = $job->getLastRun() + $job->getInterval();