From 59d7b836bf029511a7befa40423582c509dcde79 Mon Sep 17 00:00:00 2001
From: Xyene <xyene@dmoj.ca>
Date: Mon, 16 Sep 2019 18:52:49 -0400
Subject: [PATCH] Patch up race condition exposed in #478

---
 dmoj/judge.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/dmoj/judge.py b/dmoj/judge.py
index 7a673403f..b00729fa2 100644
--- a/dmoj/judge.py
+++ b/dmoj/judge.py
@@ -173,7 +173,8 @@ def grading_cleanup_wrapper():
         self.current_submission_thread.daemon = True
         self.current_submission_thread.start()
 
-        if blocking:
+        # Submission may have been killed already, but block if it hasn't been.
+        if blocking and self.current_submission_thread is not None:
             self.current_submission_thread.join()
 
     def grade_cases(self, grader, cases, short_circuit=False, is_short_circuiting=False):