test_interpreters fails when running tests sequentially

[Yhg1s]

Bug report

Bug description:

test_interpreters fails when running tests sequentially (which we do as part of the release process). The easiest reproducer seems to be python -m test test_interpreters test_interpreters.test_channels:

% bin/python -m test test_interpreters test_interpreters.test_channels
0:00:00 load avg: 0.86 [1/2] test_interpreters
0:00:02 load avg: 0.87 [2/2] test_interpreters.test_channels
test test_interpreters.test_channels crashed -- Traceback (most recent call last):
  File "/tmp/testinstall/lib/python3.13/test/libregrtest/single.py", line 178, in _runtest_env_changed_exc
    _load_run_test(result, runtests)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/tmp/testinstall/lib/python3.13/test/libregrtest/single.py", line 125, in _load_run_test
    test_mod = importlib.import_module(module_name)
               ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/tmp/testinstall/lib/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1014, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/tmp/testinstall/lib/python3.13/test/test_interpreters/test_channels.py", line 10, in <module>
    from test.support.interpreters import channels
  File "/tmp/testinstall/lib/python3.13/test/support/interpreters/channels.py", line 171, in <module>
    _channels._register_end_types(SendChannel, RecvChannel)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: already registered

The cause is libregrtest's behaviour of unloading newly imported modules from sys.modules after each test run. Either test.support.interpreters.channels should support being imported multiple times (perhaps by not doing any setup at import time, but via deliberate calls?), or libregrtest should be taught to skip unloading test.support.interpreters modules.

@ericsnowcurrently, since this seems to be your area, any opinion making test.support.interpreters do the right thing here?

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-115490: Make the interpreter.channels and interpreter.queues Modules Handle Reloading Properly #115493
• gh-115490: Work around test.support.interpreters.channels not handling unloading #115515

[ericsnowcurrently]

Yeah, I see what the problem is.

[ericsnowcurrently]

gh-115493 should take care of it. (I double-checked locally and it fixed it for me.)

[encukou]

With the workaround, this is no longer a blocker.