#Issue3088 in-progress: Race condition with instances of classes derived from threading.local:

When a thread touches such an object for the first time, a new thread-local __dict__ is created,
and the __init__ method is run.
But a thread switch can occur here; if the other thread touches the same object, it installs another
__dict__; when the first thread resumes, it updates the dictionary of the second...

This is the deep cause of the failures in test_multiprocessing involving "managers" objects.

Also a 2.5 backport candidate.

Lib/test/test_threading_local.py

@@ -42,6 +42,32 @@ class ThreadingLocalTest(unittest.TestCase):
         deadlist = [weak for weak in weaklist if weak() is None]
         self.assert_(len(deadlist) in (n-1, n), (n, len(deadlist)))
 
+    def test_derived(self):
+        # Issue 3088: if there is a threads switch inside the __init__
+        # of a threading.local derived class, the per-thread dictionary
+        # is created but not correctly set on the object.
+        # The first member set may be bogus.
+        import time
+        class Local(threading.local):
+            def __init__(self):
+                time.sleep(0.01)
+        local = Local()
+
+        def f(i):
+            local.x = i
+            # Simply check that the variable is correctly set
+            self.assertEqual(local.x, i)
+
+        threads= []
+        for i in range(10):
+            t = threading.Thread(target=f, args=(i,))
+            t.start()
+            threads.append(t)
+
+        for t in threads:
+            t.join()
+
+
 def test_main():
     suite = unittest.TestSuite()
     suite.addTest(DocTestSuite('_threading_local'))

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 2.6 beta 2?
 Core and Builtins
 -----------------
 
+- Issue #3088: Corrected a race condition in classes derived from
+  threading.local: the first member set by a thread could be saved in
+  another thread's dictionary.
+
 - Issue #3004: Minor change to slice.indices(): the start and stop
   arguments are now treated identically, making the behaviour easier
   to describe and understand.  For example, slice(None, -10,

Modules/threadmodule.c

@@ -293,7 +293,10 @@ _ldict(localobject *self)
 		}
 		
 	}
-	else if (self->dict != ldict) {
+
+	/* The call to tp_init above may have caused another thread to run.
+	   Install our ldict again. */
+	if (self->dict != ldict) {
 		Py_CLEAR(self->dict);
 		Py_INCREF(ldict);
 		self->dict = ldict;