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gh-158336: Fix race in lock_tests test_set_and_clear #158337

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Merged
vstinner merged 4 commits into from
Nov 9, 2026
+7 −2
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gh-158336: Fix race in lock_tests test_set_and_clear #158337

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gh-158336: Fix race in lock_tests test_set_and_clear
matthiasgoergens Sep 28, 2026
67a224f
Apply suggestion from @vstinner
vstinner Nov 9, 2026
106d1fa
Update Lib/test/lock_tests.py
vstinner Nov 9, 2026
484dfbf
Merge branch 'main' into fix-test-set-and-clear-flake
vstinner Nov 9, 2026
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9 changes: 7 additions & 2 deletions Lib/test/lock_tests.py
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Original file line number Diff line number Diff line change
Expand Up @@ -566,6 +566,8 @@ def test_nan_timeout(self):
evt = self.eventtype()
self.assertRaises(ValueError, evt.wait, float('nan'))

# Use private attributes (event._cond._waiters)
@support.cpython_only
def test_set_and_clear(self):
# gh-57711: check that wait() returns true even when the event is
# cleared before the waiting thread is woken up.
Expand All @@ -576,8 +578,11 @@ def f():

N = 5
with Bunch(f, N):
# Threads blocked on event.wait()
wait_threads_blocked(N)
# Wait until all threads are registered as waiters in
# event.wait().
for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
if len(event._cond._waiters) >= N:

@vstinner vstinner Sep 30, 2026

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If possible, I would prefer to avoid accessing private attributes.

Can you try instead to test if len(results) >= N: break?

@YvesDup YvesDup Oct 2, 2026

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IMO, test might be something like below:

    def test_set_and_clear(self):
        # gh-57711: check that wait() returns true even when the event is
        # cleared before the waiting thread is woken up.
        event = self.eventtype()
        N = 5
        ready = []
        results = []
        def f():
            ready.append(True)
            results.append(event.wait(support.LONG_TIMEOUT))

        with Bunch(f, N):
            # Threads blocked on event.wait()
            for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
                if len(ready) >= N:
                    break

            # Threads unblocked
            event.set()
            for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
                if len(results) >= N:
                    break
            event.clear()

        self.assertEqual(results, [True] * N)

@vstinner vstinner Oct 2, 2026

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with Bunch(...): already waits until all started threads complete. Is it really needed to wait until len(results) >= N before calling event.clear()?

The test description says:

check that wait() returns true even when the event is cleared before the waiting thread is woken up.

Waiting for theads between set() and clear() seems to go against the test description.

@YvesDup YvesDup Oct 2, 2026

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You are right. The assertEqual will fail if there is a missing value or a false value So we can remove the last support.sleeping_retry that tests the results.

@YvesDup YvesDup Oct 5, 2026 •
edited
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If possible, I would prefer to avoid accessing private attributes.

Is it acceptable to check from the representation string of Condition ? The last value is the number of waiting threads.

@vstinner vstinner Oct 5, 2026

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If my suggested if len(results) >= N: break test works, there is no need to query Condition internals.

@YvesDup YvesDup Oct 5, 2026 •
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When entering in the Bunch block, run your suggestion if len(results) >= N: break test hangs.
The results list is only updated when we quit the wait method in each thread, that is triggered from the call to the setmethod of Event in the main thread, later.
Query Condition internals seems necessary and the only way to proceed.

@YvesDup YvesDup Nov 9, 2026

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@vstinner , sorry to bother you, but the test_set_and_clear test still fails in CI. Could we fix this, please? The OP’s suggestion seems to me to be the only valid one.

@vstinner vstinner Nov 9, 2026

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Thanks for the reminder, I forgot about this change. So using only public attributes doesn't work, ok. So I just added @support.cpython_only marker on the test.

break

# Threads unblocked
event.set()
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