bpo-44422: Fix threading.enumerate() reentrant call (GH-26727) (GH-26738)

The threading.enumerate() function now uses a reentrant lock to
prevent a hang on reentrant call.
(cherry picked from commit 243fd01)

Co-authored-by: Victor Stinner <[email protected]>

Author: miss-islington

Lib/threading.py

@@ -750,8 +750,11 @@ class BrokenBarrierError(RuntimeError):
 def _newname(template="Thread-%d"):
     return template % _counter()
 
-# Active thread administration
-_active_limbo_lock = _allocate_lock()
+# Active thread administration.
+#
+# bpo-44422: Use a reentrant lock to allow reentrant calls to functions like
+# threading.enumerate().
+_active_limbo_lock = RLock()
 _active = {}    # maps thread id to Thread object
 _limbo = {}
 _dangling = WeakSet()
@@ -1474,7 +1477,7 @@ def _after_fork():
     # by another (non-forked) thread.  http://bugs.python.org/issue874900
     global _active_limbo_lock, _main_thread
     global _shutdown_locks_lock, _shutdown_locks
-    _active_limbo_lock = _allocate_lock()
+    _active_limbo_lock = RLock()
 
     # fork() only copied the current thread; clear references to others.
     new_active = {}

Misc/NEWS.d/next/Library/2021-06-14-23-28-17.bpo-44422.BlWOgv.rst

@@ -0,0 +1,3 @@
+The :func:`threading.enumerate` function now uses a reentrant lock to
+prevent a hang on reentrant call.
+Patch by Victor Stinner.