From: Takeshi KOMIYA <i.tkomiya@gmail.com>
Date: Tue, 5 Sep 2017 01:37:33 +0900
Subject: Fix #4006: Fix crash on parallel build

(cherry picked from commit 0cb57f19a099be2809a8fb5dc9274f7411e1caac)
---
 sphinx/util/parallel.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/sphinx/util/parallel.py b/sphinx/util/parallel.py
index 7ca4e94..9bc3c36 100644
--- a/sphinx/util/parallel.py
+++ b/sphinx/util/parallel.py
@@ -122,6 +122,7 @@ class ParallelTasks(object):
                     logger.handle(log)
                 self._result_funcs.pop(tid)(self._args.pop(tid), result)
                 self._procs[tid].join()
+                self._precvs.pop(tid)
                 self._pworking -= 1
                 break
         else: