From 3414ea9ed9fce2b9ce74e8b6c7c3b4278b526685 Mon Sep 17 00:00:00 2001
From: Jeffrey Yasskin <jyasskin@gmail.com>
Date: Sat, 23 Feb 2008 19:40:54 +0000
Subject: Prevent classes like:     class RunSelfFunction(object):         def
 __init__(self):             self.thread = threading.Thread(target=self._run) 
            self.thread.start()         def _run(self):             pass from
 creating a permanent cycle between the object and the thread by having the
 Thread delete its references to the object when it completes.

As an example of the effect of this bug, paramiko.Transport inherits from
Thread to avoid it.
---
 Lib/test/test_threading.py | 20 ++++++++++++++++++++
 Lib/threading.py           |  3 +++
 2 files changed, 23 insertions(+)

diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 4f49d7f..91f5a8b 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -8,6 +8,7 @@ import threading
 import thread
 import time
 import unittest
+import weakref
 
 # A trivial mutable counter.
 class Counter(object):
@@ -253,6 +254,25 @@ class ThreadTests(unittest.TestCase):
         finally:
             sys.setcheckinterval(old_interval)
 
+    def test_no_refcycle_through_target(self):
+        class RunSelfFunction(object):
+            def __init__(self):
+                # The links in this refcycle from Thread back to self
+                # should be cleaned up when the thread completes.
+                self.thread = threading.Thread(target=self._run,
+                                               args=(self,),
+                                               kwargs={'yet_another':self})
+                self.thread.start()
+
+            def _run(self, other_ref, yet_another):
+                pass
+
+        cyclic_object = RunSelfFunction()
+        weak_cyclic_object = weakref.ref(cyclic_object)
+        cyclic_object.thread.join()
+        del cyclic_object
+        self.assertEquals(None, weak_cyclic_object())
+
 
 class ThreadingExceptionTests(unittest.TestCase):
     # A RuntimeError should be raised if Thread.start() is called
diff --git a/Lib/threading.py b/Lib/threading.py
index 409360d..2f472b4 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -444,6 +444,9 @@ class Thread(_Verbose):
     def run(self):
         if self.__target:
             self.__target(*self.__args, **self.__kwargs)
+        # Avoid a refcycle if the thread is running a function with an
+        # argument that has a member that points to the thread.
+        del self.__target, self.__args, self.__kwargs
 
     def __bootstrap(self):
         # Wrapper around the real bootstrap code that ignores
-- 
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