`test_asyncio` `test_to_thread_concurrent` flaky

[colesbury]

The call_count counter on mock.Mock is not atomic, so updates may be lost:

cpython/Lib/test/test_asyncio/test_threads.py

Lines 32 to 41 in ab094d1

	async def test_to_thread_concurrent(self):
	func = mock.Mock()
	futs = []
	for _ in range(10):
	fut = asyncio.to_thread(func)
	futs.append(fut)
	await asyncio.gather(*futs)
	self.assertEqual(func.call_count, 10)

This leads to occasional errors like:

File "Lib/test/test_asyncio/test_threads.py", line 41, in test_to_thread_concurrent
self.assertEqual(func.call_count, 10)
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
AssertionError: 9 != 10

I think the update is not actually atomic even with the GIL because of the use of _delegating_property in Mock, but I'm not entirely sure. The failures are definitely more likely to occur in the free-threaded build.

Linked PRs

• gh-122957: Fix test flakiness in free-thread build #124039
• [3.13] gh-122957: Fix test flakiness in asyncio test in free-thread build (GH-124039) #124067

[kumaraditya303]

I can't think of good way to fix this, maybe mock should be made atomic?

[colesbury]

Seems easier to just use a regular function instead of a mock.Mock(). In other places, Python has for a long time used lists to count atomically. It's not clean, but it works:

        calls = []
        def func():
            calls.append(1)
...

        self.assertEqual(sum(calls), 10)

[lesteve]

I was able to reproduce the failures locally with a free-threaded build so I opened #124039 with the recommended fix (using a list instead of a Mock).