From c3b317762981baf203b2118a10f379505f3528b6 Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@gmail.com>
Date: Tue, 22 Aug 2017 12:37:23 +0200
Subject: [PATCH 1/3] 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.
---
 Lib/concurrent/futures/thread.py                           | 7 +++++--
 .../next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst  | 3 +++
 2 files changed, 8 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 1f0a1d4b977f316..ca579af75d549b9 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -54,8 +54,10 @@ def run(self):
 
         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)
 
@@ -148,4 +150,5 @@ def shutdown(self, wait=True):
         if wait:
             for t in self._threads:
                 t.join()
+        self._threads.clear()
     shutdown.__doc__ = _base.Executor.shutdown.__doc__
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 000000000000000..d3dbc124b606b24
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst
@@ -0,0 +1,3 @@
+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.

From 7e5abd2a882f1fa7b8084741dacd4fc1ce8f7969 Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@gmail.com>
Date: Tue, 22 Aug 2017 15:52:32 +0200
Subject: [PATCH 2/3] shutdown() now only clears threads if wait is true.

---
 Lib/concurrent/futures/thread.py                                | 2 +-
 .../next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
index ca579af75d549b9..320b2731b35f870 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -150,5 +150,5 @@ def shutdown(self, wait=True):
         if wait:
             for t in self._threads:
                 t.join()
-        self._threads.clear()
+            self._threads.clear()
     shutdown.__doc__ = _base.Executor.shutdown.__doc__
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
index d3dbc124b606b24..2e808348986a12a 100644
--- 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
@@ -1,3 +1,3 @@
 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.
+ThreadPoolExecutor.shutdown(wait=True) now also clears its threads set.

From b32a53f1e9272654e38756977326f06a6f848872 Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@gmail.com>
Date: Tue, 22 Aug 2017 15:56:52 +0200
Subject: [PATCH 3/3] Revert changes on shutdown()

---
 Lib/concurrent/futures/thread.py                                 | 1 -
 .../NEWS.d/next/Library/2017-08-22-12-44-48.bpo-31249.STPbb9.rst | 1 -
 2 files changed, 2 deletions(-)

diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
index 320b2731b35f870..0b5d5373ffdc0aa 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -150,5 +150,4 @@ def shutdown(self, wait=True):
         if wait:
             for t in self._threads:
                 t.join()
-            self._threads.clear()
     shutdown.__doc__ = _base.Executor.shutdown.__doc__
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
index 2e808348986a12a..f11a66802d1bc14 100644
--- 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
@@ -1,3 +1,2 @@
 concurrent.futures: WorkItem.run() used by ThreadPoolExecutor now breaks a
 reference cycle between an exception object and the WorkItem object.
-ThreadPoolExecutor.shutdown(wait=True) now also clears its threads set.