test_concurrent_futures.test_deadlock fails with env changed: sys.stderr was modified on s390x RHEL8 Refleaks 3.x

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s390x RHEL8 Refleaks 3.x:

Warning -- sys.stderr was modified by test.test_concurrent_futures.test_deadlock
Warning --   Before: <_io.TextIOWrapper name='<stderr>' mode='w' encoding='utf-8'>
Warning --   After:  <_io.StringIO object at 0x3ff971b6410>

build: https://buildbot.python.org/all/#/builders/75/builds/895

Linked PRs

• gh-109832: concurrent.futures test_deadlock restores sys.stderr #109887
• [3.12] gh-109832: concurrent.futures test_deadlock restores sys.stderr (GH-109887) #109892
• [3.11] gh-109832: concurrent.futures test_deadlock restores sys.stderr (GH-109887) #109893

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I suppose that sys.stderr was modified by test_error_at_task_unpickle() or by test_error_during_result_unpickle_in_result_handler().

I tried to reproduce the failure with:

./python -m test test_concurrent_futures.test_deadlock -m test_error_at_task_unpickle -m test_error_during_result_unpickle_in_result_handler --fail-env-changed  -R 3:3 -v --forever

I suppose that the bug only triggers if the system load is high.

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I reproduced the issue after 4 min 48 sec.

Terminal 1:

$ ./python -m test test_concurrent_futures.test_deadlock -m test_error_at_task_unpickle -m test_error_during_result_unpickle_in_result_handler --fail-env-changed  -R 3:3 -v --forever 
(...)
0:04:26 load avg: 13.97 [ 14] test_concurrent_futures.test_deadlock
(...)

Warning -- sys.stderr was modified by test_concurrent_futures.test_deadlock
Warning --   Before: <_io.TextIOWrapper name='<stderr>' mode='w' encoding='utf-8'>
Warning --   After:  <_io.StringIO object at 0x7fd38e74aea0> 

== Tests result: ENV CHANGED ==

All 13 tests OK.

1 test altered the execution environment:
    test_concurrent_futures.test_deadlock

Total duration: 4 min 48 sec
Total tests: run=84 (filtered)
Total test files: run=14 (filtered) env_changed=1
Result: ENV CHANGED

Terminal 2:

./python -m test -j10 -r

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Error on GHA macOS: https://github.com/python/cpython/actions/runs/6306288599/job/17121190375?pr=109871

0:13:56 load avg: 9.33 [458/463/1] test.test_concurrent_futures.test_deadlock failed (env changed) -- running (1): test_decimal (45.1 sec)
(...)
Warning -- sys.stderr was modified by test.test_concurrent_futures.test_deadlock
Warning --   Before: <_io.StringIO object at 0x10e3d3860>
Warning --   After:  <_io.StringIO object at 0x10efe2c30>