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

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Merged
merged 3 commits into from
Jun 15, 2021
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bpo-44422: Fix threading.enumerate() reentrant call #26727

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28fa7d7
bpo-44422: Fix threading.enumerate() reentrant call
vstinner Jun 14, 2021
adb0a4d
Fix typo
vstinner Jun 15, 2021
20a0d9a
Fix _after_fork()
vstinner Jun 15, 2021
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9 changes: 6 additions & 3 deletions Lib/threading.py
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Original file line number Diff line number Diff line change
Expand Up @@ -775,8 +775,11 @@ class BrokenBarrierError(RuntimeError):
def _newname(name_template):
return name_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()
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The RLock() is the right way to go to prevent self-deadlock.

We could temporarily turn GC off but that seems hard to do in a way that reliably restores the prior enabled/disabled mode.

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We could temporarily turn GC off but that seems hard to do in a way that reliably restores the prior enabled/disabled mode.

Sure, that's the other option that I considered. 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.

Another option would be to rewrite the code in C. The problem is that other functions rely on this lock, like active_count(). I would prefer to not have to rewrite "half" of threading.py in C. Using a RLock is less intrusive.

_active = {} # maps thread id to Thread object
_limbo = {}
_dangling = WeakSet()
Expand Down Expand Up @@ -1564,7 +1567,7 @@ 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 = {}
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2021-06-14-23-28-17.bpo-44422.BlWOgv.rst
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@@ -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.