Sync pre-release CPython 3.15 branch from GitHub (2026-08-17)

Summary:
Imported python/cpython `3.15.0rc1+dev` from upstream rev [`594d11a`](https://www.github.com/python/cpython/commit/594d11a5f55fc0d9631a6f14e38887fe76180d28) (committed 2026-08-17 03:58:10+00:00).

# Commit Info

- Base: (`3.15.0rc1+dev`) - [`39a0217`](https://www.github.com/python/cpython/commit/39a0217217ce4f50d8908883f6b835a53692e5f6) (commit date: 2026-08-15 09:43:48+00:00)
- Imported: (`3.15.0rc1+dev`) - [`594d11a`](https://www.github.com/python/cpython/commit/594d11a5f55fc0d9631a6f14e38887fe76180d28) (commit date: 2026-08-17 03:58:10+00:00)

# Noteworthy file changes

- Low-signal files (1 added) (NEWS.d, docs, .github)

Complete list of added/removed files: https://www.internalfb.com/intern/everpaste/?color=0&handle=GIGESC6epO8rVawDAIfkEr7eKqZ0br0LAAAz

Reviewed By: yoney

Differential Revision: D116253574

fbshipit-source-id: ff9a47cb81a1ee09ae5cdb8cc9d2e3f7750ce8bb

Author: generatedunixname1734921407115435

Lib/test/test_context.py

@@ -1282,6 +1282,31 @@ def test_hamt_gc_2(self):
 
         self.assertIsNone(ref())
 
+    def test_hamt_gc_3(self):
+        # gh-154535: the iterators must be tracked by the GC, otherwise a
+        # cycle running through one is never collected and the HAMT it
+        # holds -- and everything in it -- leaks.
+        A = HashKey(100, 'A')
+
+        container = []
+        h = hamt()
+        h = h.set(A, container)
+
+        hi = h.items()
+        self.assertTrue(gc.is_tracked(hi))
+
+        # Close the cycle: hi -> h -> container -> hi.
+        container.append(hi)
+        ref = weakref.ref(h)
+
+        del h, hi, container
+
+        gc.collect()
+        gc.collect()
+        gc.collect()
+
+        self.assertIsNone(ref())
+
     def test_hamt_in_1(self):
         A = HashKey(100, 'A')
         AA = HashKey(100, 'A')

Misc/NEWS.d/next/Core_and_Builtins/2026-08-10-11-20-00.gh-issue-155515.Lp3qWc.rst

@@ -0,0 +1,4 @@
+Track the internal HAMT iterators, which back iteration over a
+:class:`contextvars.Context`, with the garbage collector.  A reference cycle
+running through such an iterator was never collected, leaking the whole
+context it iterated over.

Python/hamt.c

@@ -2451,6 +2451,10 @@ hamt_baseiter_tp_clear(PyObject *op)
 {
     PyHamtIterator *it = (PyHamtIterator*)op;
     Py_CLEAR(it->hi_obj);
+    /* i_nodes holds borrowed pointers into the tree that hi_obj was keeping
+       alive, so the cursor must not be used again.  A negative i_level makes
+       hamt_iterator_next() report I_END without touching i_nodes. */
+    it->hi_iter.i_level = -1;
     return 0;
 }
 
@@ -2497,6 +2501,10 @@ static Py_ssize_t
 hamt_baseiter_tp_len(PyObject *op)
 {
     PyHamtIterator *it = (PyHamtIterator*)op;
+    if (it->hi_obj == NULL) {
+        /* tp_clear() ran on this iterator. */
+        return 0;
+    }
     return it->hi_obj->h_count;
 }
 
@@ -2517,6 +2525,7 @@ hamt_baseiter_new(PyTypeObject *type, binaryfunc yield, PyHamtObject *o)
 
     hamt_iterator_init(&it->hi_iter, o->h_root);
 
+    PyObject_GC_Track(it);
     return (PyObject*)it;
 }