`test_concurrent_futures`: `test_free_reference` is flaky

[colesbury]

Bug report

Seen in https://buildbot.python.org/#/builders/1368/builds/2775/steps/6/logs/stdio

The test attempts to ensure that the executor doesn't hold on to the result object, but the executor necessarily holds onto it for a brief period from when it sets the result in in the future until the variable goes out of scope.

We should use support.sleeping_retry to check if the weakref eventually (ideally quickly) becomes dead.

======================================================================
FAIL: test_free_reference (test.test_concurrent_futures.test_thread_pool.ThreadPoolExecutorTest.test_free_reference)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/ec2-user/buildbot/buildarea/3.x.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/test/test_concurrent_futures/executor.py", line 132, in test_free_reference
    self.assertIsNone(wr())
    ~~~~~~~~~~~~~~~~~^^^^^^
AssertionError: <test.test_concurrent_futures.executor.MyObject object at 0x200041b0100> is not None

----------------------------------------------------------------------

cpython/Lib/test/test_concurrent_futures/executor.py

Lines 125 to 132 in 12db452

	def test_free_reference(self):
	# Issue #14406: Result iterator should not keep an internal
	# reference to result objects.
	for obj in self.executor.map(make_dummy_object, range(10)):
	wr = weakref.ref(obj)
	del obj
	support.gc_collect() # For PyPy or other GCs.
	self.assertIsNone(wr())

cpython/Lib/concurrent/futures/thread.py

Lines 85 to 92 in 12db452

	try:
	result = ctx.run(self.task)
	except BaseException as exc:
	self.future.set_exception(exc)
	# Break a reference cycle with the exception 'exc'
	self = None
	else:
	self.future.set_result(result)

Linked PRs

• gh-130957: Use sleeping_retry in test_free_reference #130958
• [3.13] gh-130957: Use sleeping_retry in test_free_reference (GH-130958) #131091
• [3.12] gh-130957: Use sleeping_retry in test_free_reference (GH-130958) #131092

[colesbury]

You can more easily reproduce this failure by adding a time.sleep():

diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
index 909359b6487..73cce87b5dd 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -90,6 +90,7 @@ def run(self, ctx):
             self = None
         else:
             self.future.set_result(result)
+            time.sleep(1)

     __class_getitem__ = classmethod(types.GenericAlias)