Fix thread-safety in GC, type cache, and instruction cache by youknowone · Pull Request #7355 · RustPython/RustPython

…n#7355)

* Fix thread-safety in GC, type cache, and instruction cache

GC / refcount:
- Add safe_inc() check for strong()==0 in RefCount
- Add try_to_owned() to PyObject for atomic refcount acquire
- Replace strong_count()+to_owned() with try_to_owned() in GC
  collection and weakref callback paths to prevent TOCTOU races

Type cache:
- Add proper SeqLock (sequence counter) to TypeCacheEntry
- Readers spin-wait on odd sequence, validate after read
- Writers bracket updates with begin_write/end_write
- Use try_to_owned + pointer revalidation on read path
- Call modified() BEFORE attribute modification in set_attr

Instruction cache:
- Add pointer_cache (AtomicUsize array) to CodeUnits for
  single atomic pointer load/store (prevents torn reads)
- Add try_read_cached_descriptor with try_to_owned + pointer
  and version revalidation after increment
- Add write_cached_descriptor with version-bracketed writes

RLock:
- Fix release() to check is_owned_by_current_thread
- Add _release_save/_acquire_restore methods

* Fix RLock _acquire_restore tuple handling and unxfail threading test

* Align type cache seqlock writer protocol with CPython

* RLock: use single parking_lot level, track recursion manually

Instead of calling lock()/unlock() N times for recursion depth N,
keep parking_lot at 1 level and manage the count ourselves.
This makes acquire/release O(1) and matches CPython's
_PyRecursiveMutex approach (lock once + set level directly).

* Add try_to_owned_from_ptr to avoid &PyObject on stale ptrs

Use addr_of! to access ref_count directly from a raw pointer
without forming &PyObject first. Applied in type cache and
instruction cache hit paths where the pointer may be stale.

* Fix CI: spelling typo and xfail flaky test_thread_safety

- Fix "minimising" -> "minimizing" for cspell
- xfail test_thread_safety: dict iteration races with
  concurrent GC mutations in _finalizer_registry