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Interpreter shutdown can deadlock when a daemon thread holds the warnings lock #159485
Description
Originally reported by @krassowski in #158973.
A daemon thread can be stopped while holding the warnings lock. During interpreter finalization, garbage collection triggers a ResourceWarning, deadlocking the finalizing thread:
import threading
import time
import warnings
ready = threading.Event()
warnings.simplefilter("always", ResourceWarning)
class WarnAtShutdown:
def __init__(self):
self.self_ref = self # Defer __del__ until shutdown GC.
def __del__(self):
warnings.warn("late", ResourceWarning)
def showwarning(message, *args, **kwargs):
if str(message) == "hold":
ready.set()
time.sleep(0.1)
warnings.showwarning = showwarning
threading.Thread(
target=lambda: warnings.warn("hold", ResourceWarning),
daemon=True,
).start()
assert ready.wait(4)
WarnAtShutdown()Expected: the process exits normally. Actual: it hangs during interpreter shutdown.
- CPython versions tested on:
3.14.8,3.15.0. - Operating systems tested on:
macOS.
Activity
pythongh-159485: Fix warnings lock deadlock during interpreter shutdown
ngoldbaum commentedon Oct 16, 2026
I updated the reproducer script in the issue to avoid an unbounded wait (it now just does
time.sleep(0.1)liketest_threadingdoes). This is more realistic and is fixed by @ZeroIntensity's InterpreterGuard approach. There's no need to guard against waiting forever in a callback like my original reproducer was arguing for. There's still a deadlock even without that and the InterpreterGuard fixes it.pythongh-159485: Hold an interpreter guard while using the warnings lock
pythongh-159485: Use the public interpreter guard API for the warning…