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gh-146056: Fix TreeBuilder stack in xml.etree by vstinner · Pull Request #146062 · python/cpython · GitHub

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

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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 number Diff line number Diff line change
@@ -1,4 +1,5 @@
# xml.etree test for cElementTree
import gc
import io
import struct
from test import support
Expand Down Expand 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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Original file line number Diff line number Diff line change
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-storchaka Mar 18, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This can have performance impact.
if (!t->stack) {
Py_DECREF(t->this);
Py_DECREF(t->last);
Expand Down Expand 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-storchaka Mar 18, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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);
        return NULL;
    }

    return this;

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.

vstinner Mar 19, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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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