The threading.enumerate() code is simple:
---
# Active thread administration
_active_limbo_lock = _allocate_lock()
_active = {}    # maps thread id to Thread object
_limbo = {}

def enumerate():
    with _active_limbo_lock:
        return list(_active.values()) + list(_limbo.values())
---

values(), list() and list+list operations can call trigger a GC collection depending on the GC thresholds, created Python objects, etc.

During a GC collection, a Python object destructor (__del__) can again call threading.enumerate().

Problem: _active_limbo_lock is not re-entrant lock and so the second call hangs.

In practice, this issue was seen in OpenStack which uses eventlet, when a destructor uses the logging module. The logging module uses threading.current_thread(), but this function is monkey-patched by the eventlet module.

Extract of the eventlet function:
---
def current_thread():
    ...
    found = [th for th in __patched_enumerate() if th.ident == g_id]
    ...
---
https://github.com/eventlet/eventlet/blob/master/eventlet/green/threading.py

Attached PR makes _active_limbo_lock re-entrant to avoid this issue. I'm not sure if it's ok or not, I would appreciate to get a review and your feedback ;-)

Attached file triggers the threading.enumerate() hang on re-entrant call.

There are different ways to fix this issue:

* (A) Rewrite threading.enumerate() in C with code which cannot trigger a GC collection
* (B) Disable temporarily the GC
* (C) Use a reentrant lock (PR 26727)

(A) The problem is that functions other than threading.enumerate() 
also rely on this lock, like threading.active_count(). I would prefer to not have to rewrite "half" of threading.py in C. Using a RLock is less intrusive.

(B) This is a simple and reliable option. But gc.disable() is process-wide: it affects all Python threads, and so it might have surprising side effects. Some code might rely on the current exact GC behavior. I would prefer to not disable the GC temporarily.

New changeset 243fd01047ddce1a7eb0f99a49732d123e942c63 by Victor Stinner in branch 'main':
bpo-44422: Fix threading.enumerate() reentrant call (GH-26727)
https://github.com/python/cpython/commit/243fd01047ddce1a7eb0f99a49732d123e942c63

New changeset c3b776f83b4c765da6d7b8854e844e07bd33c375 by Miss Islington (bot) in branch '3.10':
bpo-44422: Fix threading.enumerate() reentrant call (GH-26727)
https://github.com/python/cpython/commit/c3b776f83b4c765da6d7b8854e844e07bd33c375

New changeset 8fe57aacc7bf9d9af84803b69dbb1d66597934c6 by Miss Islington (bot) in branch '3.9':
bpo-44422: Fix threading.enumerate() reentrant call (GH-26727) (GH-26738)
https://github.com/python/cpython/commit/8fe57aacc7bf9d9af84803b69dbb1d66597934c6

Ok, the deadlock on reentrant call to threading.enumerate() is now fixed in 3.9, 3.10 and main (future 3.11) branches. I close the issue. Thanks for the reviews, as usual :-)

Oh, I reopen the issue for PR 26741.

New changeset 0729694246174a5c2f0ae197f2e0dbea61b90c9f by Victor Stinner in branch 'main':
bpo-44422: threading.Thread reuses the _delete() method (GH-26741)
https://github.com/python/cpython/commit/0729694246174a5c2f0ae197f2e0dbea61b90c9f