From c267b5650572e9a40667200a07b939b9957879b8 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 8ac26e34a..ecaf365ed 100644
--- a/dmoj/judge.py
+++ b/dmoj/judge.py
@@ -115,7 +115,8 @@ def process_submission(self, type, target, id, *args, **kwargs):
         self.current_submission_thread.daemon = True
         self.current_submission_thread.start()
 
-        if is_blocking:
+        # Submission may have been killed already, but block if it hasn't been.
+        if is_blocking and self.current_submission_thread is not None:
             self.current_submission_thread.join()
 
     def _custom_invocation(self, language, source, memory_limit, time_limit, input_data):