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[3.14] gh-155515: Use GC tracking for HAMT iterators (GH-155517) #155698

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[3.14] gh-155515: Use GC tracking for HAMT iterators (GH-155517) #155698

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25 changes: 25 additions & 0 deletions Lib/test/test_context.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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')
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-08-10-11-20-00.gh-issue-155515.Lp3qWc.rst
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@@ -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.
9 changes: 9 additions & 0 deletions Python/hamt.c
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Expand Up @@ -2450,6 +2450,10 @@ 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;
}

Expand Down Expand Up @@ -2496,6 +2500,10 @@ 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;
}

Expand All @@ -2516,6 +2524,7 @@ 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;
}

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