From bc61315377056fe362b744d9c44e17cd3178ce54 Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@gmail.com>
Date: Tue, 22 Aug 2017 16:50:42 +0200
Subject: [PATCH] bpo-31249: Fix ref cycle in ThreadPoolExecutor (#3178)

* bpo-31249: Fix ref cycle in ThreadPoolExecutor

concurrent.futures: WorkItem.run() used by ThreadPoolExecutor now
breaks a reference cycle between an exception object and the WorkItem
object. ThreadPoolExecutor.shutdown() now also clears its threads
set.

* shutdown() now only clears threads if wait is true.

* Revert changes on shutdown()
---
 Lib/concurrent/futures/thread.py                            | 6 ++++--
 .../next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst   | 2 ++
 2 files changed, 6 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst

diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
index 1f0a1d4b977..0b5d5373ffd 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -54,8 +54,10 @@ class _WorkItem(object):
 
         try:
             result = self.fn(*self.args, **self.kwargs)
-        except BaseException as e:
-            self.future.set_exception(e)
+        except BaseException as exc:
+            self.future.set_exception(exc)
+            # Break a reference cycle with the exception 'exc'
+            self = None
         else:
             self.future.set_result(result)
 
diff --git a/Misc/NEWS.d/next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst b/Misc/NEWS.d/next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst
new file mode 100644
index 00000000000..f11a66802d1
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst
@@ -0,0 +1,2 @@
+concurrent.futures: WorkItem.run() used by ThreadPoolExecutor now breaks a
+reference cycle between an exception object and the WorkItem object.