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gh-146056: Fix TreeBuilder stack in xml.etree#146062

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vstinner wants to merge 1 commit into from
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gh-146056: Fix TreeBuilder stack in xml.etree #146062

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18 changes: 18 additions & 0 deletions Lib/test/test_xml_etree_c.py
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Original file line numberDiff line numberDiff line change
@@ -1,4 +1,5 @@
# xml.etree test for cElementTree
import gc
import io
import struct
from test import support
Expand DownExpand Up@@ -203,6 +204,23 @@ def test_disallow_instantiation(self):
iter_type = type(root.iter())
support.check_disallow_instantiation(self, iter_type)

def test_tree_builder_stack(self):
builder = cET.TreeBuilder()
for x in range(10):
builder.start("a", {})
for x in range(10):
builder.end("a")
root = builder.close()

# Find the internal TreeBuilder stack list using gc.get_referrers()
for ref in gc.get_referrers(root[0]):
if isinstance(ref, list):
# Check that the list doesn't contain NULL items (gh-146056)
repr(ref)
break
else:
self.fail("failed to find TreeBuilder stack list")


@unittest.skipUnless(cET, 'requires _elementtree')
class TestAliasWorking(unittest.TestCase):
Expand Down
6 changes: 3 additions & 3 deletions Modules/_elementtree.c
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Expand Up@@ -2431,7 +2431,7 @@ treebuilder_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
t->element_factory = NULL;
t->comment_factory = NULL;
t->pi_factory = NULL;
t->stack = PyList_New(20);
t->stack = PyList_New(0);

@serhiy-storchakaserhiy-storchakaMar 18, 2026

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This can have performance impact.

if (!t->stack) {
Py_DECREF(t->this);
Py_DECREF(t->last);
Expand DownExpand Up@@ -2856,9 +2856,9 @@ treebuilder_handle_end(TreeBuilderObject* self, PyObject* tag)

item = self->last;
self->last = Py_NewRef(self->this);
Py_XSETREF(self->last_for_tail, self->last);
Py_XSETREF(self->last_for_tail, Py_NewRef(self->last));

@serhiy-storchakaserhiy-storchakaMar 18, 2026

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This is unrelated.

If rewrite this code to be more explicit or safe, I would write something like

PyObject*last=self->last;
PyObject*last_for_tail=self->last_for_tail;
PyObject*this=self->this;
self->index--;
self->this=Py_NewRef(PyList_GET_ITEM(self->stack, self->index));
self->last=Py_NewRef(this);
self->last_for_tail=Py_NewRef(this);
Py_DECREF(last);
Py_XDECREF(last_for_tail);
if (treebuilder_append_event(self, self->end_event_obj, this) <0) {
Py_DECREF(this);
returnNULL;
}
returnthis;

But we should also look a the other ends -- how these attributes are set in other code in this file. This is a separate issue.

@vstinnervstinnerMar 19, 2026

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I wrote #146167 which uses your suggestion.

self->index--;
self->this = Py_NewRef(PyList_GET_ITEM(self->stack, self->index));
Py_SETREF(self->this, Py_NewRef(PyList_GET_ITEM(self->stack, self->index)));
Py_DECREF(item);

if (treebuilder_append_event(self, self->end_event_obj, self->last) < 0)
Expand Down
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