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()

Author: vstinner

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)
 

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.