crossinterp.c: Use-after-free + leaked exception + shadowed variable

Bug report

Bug description:

Original gist: https://gist.github.com/devdanzin/a56b8cb579b1950b4206e8a42d6be650

Summary

Three bugs in the cross-interpreter subsystem:

1. _ensure_notshareableerror (crossinterp_exceptions.h:85): Missing return after _PyErr_SetRaisedException steals ctx ref. Falls through to code using freed pointer.
2. _PyXI_UnwrapNotShareableError (crossinterp_data_lookup.h:102): Exception removed from thread state via _PyErr_GetRaisedException, then _PyXI_InitFailure fails → exception leaked, thread error indicator cleared.
3. _pop_preserved (crossinterp.c:2968): Inner _PyXI_namespace *xidata shadows outer. goto error cleanup checks outer (NULL) variable → inner allocation leaked.

Reproducer (ref leak)

import _interpchannels as ch, sys

cid = ch.create()
before = sys.gettotalrefcount()
for i in range(10000):
    try:
        ch.send(cid, lambda: None, blocking=False)
    except Exception:
        pass
after = sys.gettotalrefcount()
ch.destroy(cid)
print(f"Leaked {after - before} refs (~{(after-before)//10000}/send)")

The third issue is already fixed by https://github.com/python/cpython/pull/147999

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-153665

Start with crossinterp_exceptions.h:85, crossinterp_data_lookup.h:102, and crossinterp.c:2968, then run the supplied _interpchannels reference-count reproducer on CPython main. Check the existing PRs, since the shadowed-variable issue is already reported as fixed; done means the two remaining exception and reference-management bugs are corrected without leaks or use-after-free behavior.