gh-123471: Make concurrent iteration over itertools.repeat safe under free-threading

[eendebakpt]

If needed we can add a unit test similar to the one in https://github.com/python/cpython/pull/131212/files

• Issue: Make concurrent iteration over pairwise, combinations, permutations, cwr, product, etc. from itertools safe under free-threading #123471

[rhettinger]

Is it possible to create tests demonstrating code the previously failed that can now be counted on to succeed?

[eendebakpt]

@rhettinger The issue that this PR solves is the following: when defining the iterator as repeat(data, 245) and concurrent iteration with multiple threads in this part of the code:

 if (ro->cnt > 0)
ro->cnt--;

we can have multiple threads passing the condition ro->cnt > 0 (if ro->cnt is 1), and then decrementing ro->cnt to a negative value. The iterator will then nevert be exhausted (even though this is not the intend of the user).

A test for this is:

import unittest
from threading import Thread, Barrier
from itertools import repeat
from test.support import threading_helper
threading_helper.requires_working_threading(module=True)
class ItertoolsThreading(unittest.TestCase):
@threading_helper.reap_threads
def test_repeat(self):
number_of_threads = 20
number_of_iterations = 700_000
barrier = Barrier(number_of_threads)
def work(it):
barrier.wait()
while True:
try:
_ = next(it)
except StopIteration:
break
data = 3
for it in range(number_of_iterations):
print(f'iteration {it}')
repeat_iterator = repeat(data, 245)
worker_threads = []
for ii in range(number_of_threads):
worker_threads.append(
Thread(target=work, args=[repeat_iterator]))
with threading_helper.start_threads(worker_threads):
pass
barrier.reset()
if __name__ == "__main__":
unittest.main()

On my system this creates a deadlock (typically after a couple of 1000) iterations in a free-threading debug build.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot iOS ARM64 Simulator 3.x (tier-3) has failed when building commit 9d127e8.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1380/builds/3222) and take a look at the build logs.
4. Check if the failure is related to this commit (9d127e8) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1380/builds/3222

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
File "/Users/buildbot/Library/Developer/XCTestDevices/7817F54E-F383-435E-A96B-7848C5D4910E/data/Containers/Bundle/Application/6FD8422A-BA2D-42A0-9F8A-786BED06BB0E/iOSTestbed.app/python/lib/python3.14/threading.py", line 1079, in _bootstrap_innerself._context.run(self.run)
~~~~~~~~~~~~~~~~~^^^^^^^^^^
File "/Users/buildbot/Library/Developer/XCTestDevices/7817F54E-F383-435E-A96B-7848C5D4910E/data/Containers/Bundle/Application/6FD8422A-BA2D-42A0-9F8A-786BED06BB0E/iOSTestbed.app/python/lib/python3.14/threading.py", line 1021, in runself._target(*self._args, **self._kwargs)
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/buildbot/Library/Developer/XCTestDevices/7817F54E-F383-435E-A96B-7848C5D4910E/data/Containers/Bundle/Application/6FD8422A-BA2D-42A0-9F8A-786BED06BB0E/iOSTestbed.app/python/lib/python3.14/test/test_interpreters/test_stress.py", line 30, in task
interp = interpreters.create()
File "/Users/buildbot/Library/Developer/XCTestDevices/7817F54E-F383-435E-A96B-7848C5D4910E/data/Containers/Bundle/Application/6FD8422A-BA2D-42A0-9F8A-786BED06BB0E/iOSTestbed.app/python/lib/python3.14/test/support/interpreters/__init__.py", line 76, in createid= _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
k

[freakboy3742]

The iOS failure is caused by #127108