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.