Issue #7264: Fix a possible deadlock when deallocating thread-local objects

which are part of a reference cycle.

Lib/_threading_local.py

@@ -218,10 +218,12 @@ class local(_localbase):
         key = object.__getattribute__(self, '_local__key')
 
         try:
-            threads = list(threading.enumerate())
+            # We use the non-locking API since we might already hold the lock
+            # (__del__ can be called at any point by the cyclic GC).
+            threads = threading._enumerate()
         except:
-            # If enumerate fails, as it seems to do during
-            # shutdown, we'll skip cleanup under the assumption
+            # If enumerating the current threads fails, as it seems to do
+            # during shutdown, we'll skip cleanup under the assumption
             # that there is nothing to clean up.
             return
 

Lib/threading.py

@@ -810,6 +810,10 @@ def activeCount():
 
 active_count = activeCount
 
+def _enumerate():
+    # Same as enumerate(), but without the lock. Internal use only.
+    return _active.values() + _limbo.values()
+
 def enumerate():
     with _active_limbo_lock:
         return _active.values() + _limbo.values()

Misc/NEWS

@@ -426,6 +426,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #7264: Fix a possible deadlock when deallocating thread-local objects
+  which are part of a reference cycle.
+
 - Issue #7211: Allow 64-bit values for the `ident` and `data` fields of kevent
   objects on 64-bit systems.  Patch by Michael Broghton.