gh-139400: Make sure that parent parsers outlive their subparsers in pyexpat

Lib/test/test_pyexpat.py

@@ -771,6 +771,42 @@ def resolve_entity(context, base, system_id, public_id):
  self.assertEqual(handler_call_args, [("bar", "baz")])
 
 
+class ParentParserLifetimeTest(unittest.TestCase):
+ """
+ Subparsers make use of their parent XML_Parser inside of Expat.
+ As a result, parent parsers need to outlive subparsers.
+
+ See https://github.com/python/cpython/issues/139400.
+ """
+
+ def test_parent_parser_outlives_its_subparsers__single(self):
+ parser = expat.ParserCreate()
+ subparser = parser.ExternalEntityParserCreate(None)
+
+ # Now try to cause garbage collection of the parent parser
+ # while it's still being referenced by a related subparser.
+ del parser
+
+ def test_parent_parser_outlives_its_subparsers__multiple(self):
+ parser = expat.ParserCreate()
+ subparser_one = parser.ExternalEntityParserCreate(None)
+ subparser_two = parser.ExternalEntityParserCreate(None)
+
+ # Now try to cause garbage collection of the parent parser
+ # while it's still being referenced by a related subparser.
+ del parser
+
+ def test_parent_parser_outlives_its_subparsers__chain(self):
+ parser = expat.ParserCreate()
+ subparser = parser.ExternalEntityParserCreate(None)
+ subsubparser = subparser.ExternalEntityParserCreate(None)
+
+ # Now try to cause garbage collection of the parent parsers
+ # while they are still being referenced by a related subparser.
+ del parser
+ del subparser
+
+
 class ReparseDeferralTest(unittest.TestCase):
  def test_getter_setter_round_trip(self):
  parser = expat.ParserCreate()

Misc/NEWS.d/next/Core_and_Builtins/2025-09-29-00-01-28.gh-issue-139400.X2T-jO.rst

@@ -0,0 +1,4 @@
+:mod:`xml.parsers.expat`: Make sure that parent Expat parsers are only
+garbage-collected once they are no longer referenced by subparsers created
+by :meth:`~xml.parsers.expat.xmlparser.ExternalEntityParserCreate`.
+Patch by Sebastian Pipping.

Modules/pyexpat.c

@@ -76,6 +76,15 @@ typedef struct {
  PyObject_HEAD
 
  XML_Parser itself;
+ /*
+ * Strong reference to a parent `xmlparseobject` if this parser
+ * is a child parser. Set to NULL if this parser is a root parser.
+ * This is needed to keep the parent parser alive as long as it has
+ * at least one child parser.
+ *
+ * See https://github.com/python/cpython/issues/139400 for details.
+ */
+ PyObject *parent;
  int ordered_attributes; /* Return attributes as a list. */
  int specified_attributes; /* Report only specified attributes. */
  int in_callback; /* Is a callback active? */
@@ -1065,6 +1074,11 @@ pyexpat_xmlparser_ExternalEntityParserCreate_impl(xmlparseobject *self,
  return NULL;
  }
 
+ // The new subparser will make use of the parent XML_Parser inside of Expat.
+ // So we need to take subparsers into account with the reference counting
+ // of their parent parser.
+ Py_INCREF(self);
+
  new_parser->buffer_size = self->buffer_size;
  new_parser->buffer_used = 0;
  new_parser->buffer = NULL;
@@ -1074,18 +1088,21 @@ pyexpat_xmlparser_ExternalEntityParserCreate_impl(xmlparseobject *self,
  new_parser->ns_prefixes = self->ns_prefixes;
  new_parser->itself = XML_ExternalEntityParserCreate(self->itself, context,
  encoding);
+ new_parser->parent = (PyObject *)self;
  new_parser->handlers = 0;
  new_parser->intern = Py_XNewRef(self->intern);
 
  if (self->buffer != NULL) {
  new_parser->buffer = PyMem_Malloc(new_parser->buffer_size);
  if (new_parser->buffer == NULL) {
  Py_DECREF(new_parser);
+ Py_DECREF(self);
  return PyErr_NoMemory();
  }
  }
  if (!new_parser->itself) {
  Py_DECREF(new_parser);
+ Py_DECREF(self);
  return PyErr_NoMemory();
  }
 
@@ -1099,6 +1116,7 @@ pyexpat_xmlparser_ExternalEntityParserCreate_impl(xmlparseobject *self,
  new_parser->handlers = PyMem_New(PyObject *, i);
  if (!new_parser->handlers) {
  Py_DECREF(new_parser);
+ Py_DECREF(self);
  return PyErr_NoMemory();
  }
  clear_handlers(new_parser, 1);
@@ -1479,6 +1497,7 @@ newxmlparseobject(pyexpat_state *state, const char *encoding,
  /* namespace_separator is either NULL or contains one char + \0 */
  self->itself = XML_ParserCreate_MM(encoding, &ExpatMemoryHandler,
  namespace_separator);
+ self->parent = NULL;
  if (self->itself == NULL) {
  PyErr_SetString(PyExc_RuntimeError,
  "XML_ParserCreate failed");
@@ -1515,6 +1534,7 @@ xmlparse_traverse(PyObject *op, visitproc visit, void *arg)
  for (size_t i = 0; handler_info[i].name != NULL; i++) {
  Py_VISIT(self->handlers[i]);
  }
+ Py_VISIT(self->parent);
  Py_VISIT(Py_TYPE(op));
  return 0;
 }
@@ -1525,6 +1545,10 @@ xmlparse_clear(PyObject *op)
  xmlparseobject *self = xmlparseobject_CAST(op);
  clear_handlers(self, 0);
  Py_CLEAR(self->intern);
+ // NOTE: We cannot call Py_CLEAR(self->parent) prior to calling
+ // XML_ParserFree(self->itself), or a subparser could lose its parent
+ // XML_Parser while still making use of it internally.
+ // https://github.com/python/cpython/issues/139400
  return 0;
 }
 
@@ -1538,6 +1562,7 @@ xmlparse_dealloc(PyObject *op)
  XML_ParserFree(self->itself);
  }
  self->itself = NULL;
+ Py_CLEAR(self->parent);
 
  if (self->handlers != NULL) {
  PyMem_Free(self->handlers);