Lack of synchronization in iterators

“Sharing `PySeqIter` across threads use-after-frees the sequence under free-threading # Crash report ### What happened? On a free-threaded build, advancing a single, shared iterator over a legacy sequence (a type implementing `__getitem__`/`sq_item` but not `__iter__`/`tp_iter`) from multiple threads use-after-frees the underlying sequence object. The defect is in the fallback iterator itself, so it affects pure-Python classes and C extension types alike. `list` (whose `list_iterator` was hardened for free-threading in https://github.com/python/cpython/pull/115605) is clean. `iter_iternext()` in `Objects/iterobject.c` has no synchronization: https://github.com/python/cpython/blob/9cbd578e38a5ef59f9462b61d1e03338b208cba6/Objects/iterobject.c#L52-L83 The iterator owns exactly one reference to `it_seq` and releases it exactly once, on exhaustion. With the GIL that invariant holds because only one thread is ever inside `tp_iternext`. Without the GIL: 1. **Double DECREF on exhaustion.** Several threads read the same non-`NULL` `it->it_seq`, all observe out-of-bounds from `PySequence_GetItem`, and each executes the `Py_DECREF(seq)`. The single owned reference is released N times, so the sequence can be freed while other code is still holding refs to it. 2. **Borrowed pointer outliving the object.** Thread A is inside `PySequence_GetItem(seq, ...)` (an arbitrarily long call) and thread B takes the exhaustion path and drops what may be the last reference. A then operates on freed memory. (There's also a data race for `index` that #115605 fixed for lists with atomic stores, but per #124397, that's acceptable.) `calliter_iternext` has the same defect: `it_callable`/`it_sentinel` are cleared with non-atomic `Py_CLEAR` on the exhaustion path and read without protection, so a shared `callable_iterator` can double-release and use-after-free them the same way. There's prior art for this issue class in #154043, #154108, #154130 and the same fixes can be applied here. ### Reproducer ```python import sys, threading THREADS = 16 ROUNDS = 2000 class PySeq: # sq_item only -> iter() falls back to PySeqIter def __init__(self, n): self.n = n def __len__(self): return self.n def __getitem__(self, i): if i >= self.n: raise IndexError(i) return i def drain(it, barrier): barrier.wait() while True: try: next(it) except StopIteration: return except Exception: return # torn it_index can walk off the end; benign def hammer(make): for _ in range(ROUNDS): it = make() barrier = threading.Barrier(THREADS) ws = [ threading.Thread(target=drain, args=(it, barrier)) for _ in range(THREADS) ] for w in ws: w.start() for w in ws: w.join() del it seq = PySeq(4) print("gil enabled:", sys._is_gil_enabled()) print("iter type:", type(iter(seq)).__name__) before = sys.getrefcount(seq) hammer(lambda: iter(seq)) after = sys.getrefcount(seq) print( f"before={before} after={after} -> {'CORRUPTED' if after != before else 'ok'}" ) ``` #### Observed results All on macOS / arm64 (Apple Silicon): | Build | `PySeq` (`PySeqIter`) | `[0, 1, 2, 3]` (`list_iterator`) | | --- | --- | --- | | main (`3.16.0a0`, free-threading debug build) | SIGSEGV | clean, refcount ok | | 3.14.6 free-threaded | SIGSEGV (3/3 runs) | clean (3/3 runs) | | 3.13.8 free-threaded | SIGSEGV (3/3 runs) | not run | | 3.14.6 GIL-enabled | clean, refcount ok | clean | | 3.15.x free-threaded | refcount bad (on some runs; see below) | not run | Depending on allocator timing the failure may show up as refcount corruption, type confusion in unrelated code once the freed memory is reused, or as SIGSEGV, or nothing at all: ``` ~/Desktop $ uv run --python=3.15t --isolated repro.py gil enabled: False iter type: iterator ~/Desktop $ uv run --python=3.15t --isolate”