Possible lost notification in threading.Condition when a waiter times out

Bug report

Hi,

I noticed some unexpected behavior while testing threading.Condition on Python 3.14.4.

I created two threads waiting on the same condition. The first thread has a short timeout, while the second has a longer timeout. I then held the condition lock until the first waiter's timeout expired and called notify(1).

In my test, both wait() calls returned False, even though I called notify(1) while two waiters were still present in the internal wait queue. I observed the same result in 100 consecutive runs.

I'm not sure whether this is expected behavior or a race condition in how timed-out waiters are handled. Could someone clarify whether this behavior is expected? If not, it may be possible for a notification to be consumed by a waiter whose timeout has already expired, leaving another waiter waiting until its own timeout.

I can provide the minimal reproduction script and terminal output to help investigate.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

gh-160414: pass a notification on when Condition.wait() times out #160503

import sys
import threading
import time

def poc_condition_notify_lost():
cond = threading.Condition()
res = {}

t1_in = threading.Event()
t2_in = threading.Event()

def w1():
    with cond:
        t1_in.set()
        res["t1"] = cond.wait(timeout=0.02)

def w2():
    t1_in.wait()
    with cond:
        t2_in.set()
        res["t2"] = cond.wait(timeout=0.2)

a = threading.Thread(target=w1)
b = threading.Thread(target=w2)

a.start()
b.start()
t2_in.wait()

with cond:
    time.sleep(0.05)
    cond.notify(1)

a.join()
b.join()

print("Python:", sys.version)
print("Results:", res)

if res == {"t1": False, "t2": False}:
    print("Observed: both wait calls returned False")
else:
    print("Observed:", res)

if name == "main":
poc_condition_notify_lost()

Thanks for the bug report!

pythongh-160414: make `test_notify_after_timeout` deterministic

Reopening for the pending fix.

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Possible lost notification in threading.Condition when a waiter times out #160414

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Possible lost notification in threading.Condition when a waiter times out #160414

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Bug report

CPython versions tested on:

Operating systems tested on:

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0x-bala commented on Oct 23, 2026

NekoAsakura commented on Oct 23, 2026

NekoAsakura commented on Oct 24, 2026

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