getargs.c is Breaking Interpeter Isolation

[ericsnowcurrently]

Bug report

Bug description:

@1st1 encountered some crashes due to this.

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

No response

Linked PRs

• [3.12] gh-119213: Fix getargs.c to store state in InterpreterState... #119194
• gh-119213: Fix getargs.c to store state in InterpreterState... #119195
• gh-119213: Be More Careful About _PyArg_Parser.kwtuple Across Interpreters #119331
• [3.13] gh-119213: Be More Careful About _PyArg_Parser.kwtuple Across Interpreters (gh-119331) #119410
• [3.12] gh-119213: Be More Careful About _PyArg_Parser.kwtuple Across Interpreters (gh-119331) #119425

[ericsnowcurrently]

Here's what I've found:

• there's a bug (in 3.13/main) where _PyRuntime.getargs.static_parsers is only ever set to the last node added by _parser_init()
• for non-builtin modules (even for Py_BUILD_CORE_MODULE) _PyArg_Parser.kwtuple is allocated on the heap under the current interpreter

The first bug masks the problem on 3.13/main, so thankfully @1st1 was testing on 3.12.

The second bug is solvable by either always statically allocating the kwtuple or by always temporarily switching to the main interpreter when allocating from the heap.

Now that I can reliably reproduce the crash I'll have a fix up soon.

[ericsnowcurrently]

One other note:

If I destroy the interpreter before runtime finalization I hit an assertion about negative refcount:

./Include/object.h:1040: _Py_NegativeRefcount: Assertion failed: object has negative ref count
<object at 0x7f6922fd5800 is freed>
Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: finalizing (tstate=0x000055d542ee5cb0)

Current thread 0x00007f692773b100 (most recent call first):
  <no Python frame>
Aborted (core dumped)

If I do not destroy the subinterpreter before runtime fini then it crashes due to the dangling pointer:

Debug memory block at address p=0x7f06a7ad4620: API ''
    71492436437630461 bytes originally requested
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfd):
        at p-7: 0x00 *** OUCH
        at p-6: 0x00 *** OUCH
        at p-5: 0x00 *** OUCH
        at p-4: 0x00 *** OUCH
        at p-3: 0x00 *** OUCH
        at p-2: 0x00 *** OUCH
        at p-1: 0x00 *** OUCH
    Because memory is corrupted at the start, the count of bytes requested
       may be bogus, and checking the trailing pad bytes may segfault.
    The 8 pad bytes at tail=0xfe7d04a5ab441d are Segmentation fault (core dumped)

[ericsnowcurrently]

FTR, original change: gh-95860