Reference leaks on current main

[Eclips4]

Bug report

Bug description:

Noted by @picnixz in #119827 (comment):

Failed tests:

    test__interpchannels
    test_capi
    test__interpreters
    test_ast
    test_importlib

Test leaking resources:

    test_importlib: references
    test_ast: references
    test__interpchannels: references
    test__interpreters: memory blocks
    test_capi: memory blocks
    test_capi: references
    test__interpreters: references
    test_importlib: memory blocks
    test_ast: memory blocks
    test__interpchannels: memory blocks

I'm bisecting a bad commit right now.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

[Eclips4]

Bisected to d929652.
Seems related to handling immortal objects in subinterpreters.

[ZeroIntensity]

I don't think this is related to subinterpreters, but more that the Placeholder singleton isn't stored elsewhere, so it's never freed. It just gets reinitialized for every subinterpreter, so the leak seems greater.

I can try and fix this much later today, but anyone feel free to try and do it before me.

[vstinner]

The issue can be reproduced with:

$ ./python -m test -R 3:3 test_ast -m test_subinterpreter
(...)
test_ast leaked [106, 106, 106] references, sum=318
(...)

which can be simplified to:

    def test_subinterpreter(self):
        code = dedent('import _functools')
        res = support.run_in_subinterp(code)
        self.assertEqual(res, 0)

[vstinner]

I wrote PR gh-124601 to fix the leak.

[vstinner]

The leak was fixed by #124601.