Indicate to the judgehost to retry if we are cleaning up old queue ta…

judge/judgedaemon.main.php

@@ -60,6 +60,7 @@ function read_credentials(): void
             "waiting" => false,
             "errorred" => false,
             "last_attempt" => -1,
+            "retrying" => false,
         ];
     }
     if (count($endpoints) <= 0) {
@@ -784,8 +785,19 @@ function fetch_executable_internal(
 
     // We have gotten a work packet.
     $endpoints[$endpointID]["waiting"] = false;
+
     // All tasks are guaranteed to be of the same type.
     $type = $row[0]['type'];
+
+    if ($type == 'try_again') {
+        if (!$endpoints[$endpointID]['retrying']) {
+            logmsg(LOG_INFO, "API indicated to retry fetching work (this might take a while to clean up).");
+        }
+        $endpoints[$endpointID]['retrying'] = true;
+        continue;
+    }
+    $endpoints[$endpointID]['retrying'] = false;
+
     logmsg(LOG_INFO,
         "⇝ Received " . sizeof($row) . " '" . $type . "' judge tasks (endpoint $endpointID)");
 

webapp/src/Controller/API/JudgehostController.php

@@ -1594,8 +1594,16 @@ public function getJudgeTasksAction(Request $request): array
                 ->setMaxResults(1)
                 ->getQuery()
                 ->getOneOrNullResult(AbstractQuery::HYDRATE_SINGLE_SCALAR);
+            if ($jobid === null) {
+                return;
+            }
             $judgetasks = $this->getJudgetasks($jobid, $max_batchsize, $judgehost);
-            if ($judgetasks !== null) {
+            if (empty($judgetasks)) {
+                // Somehow we got ourselves in a situation that there was a queue task without remaining judge tasks.
+                // This should not happen, but if it does, we need to clean up. Each of the fetch-work calls will clean
+                // up one queue task. We need to signal to the judgehost that there might be more work to do.
+                $judgetasks = [['type' => 'try_again']];
+            } else {
                 // Mark it as being worked on.
                 $this->em->createQueryBuilder()
                     ->update(QueueTask::class, 'qt')